"Why was Sun Wukong unstoppable when he stormed the Heavens, but couldn't beat a few monsters on the journey?" This is a question that I get asked a lot by followers, and have seen many others online who, too, are questioning this topic.
1. Why Monkey Seems Less Capable
When discussing why Wukong seems weaker on the journey, all of the reasons can be classified into two categories. This is the first category; it focuses on explanations as to why Wukong always seems to lose in duels against demons on the journey, when he had the potential to wreak havoc in the celestial palace. It covers possibilities why Monkey’s powers may have decreased or why the demons simply appear to be stronger.
1.1. Lack of Exercise
It’s common knowledge that the Monkey King was ultimately subdued by the Tathagata Buddha and was pinned under Five-Elements Mountain to serve imprisonment for his crimes against Heaven [src. 2]. This imprisonment lasted for over 600 years , restricting his freedom of movement [src. 3]. Because of this factor, it was not possible for Sun Wukong to practise his skills, and gradually over time, they would decline. In addition to his lack of practice, the demons on the journey had all this time of 600 years to cultivate their own skills and expand their powers.
“Tathagata deceived me,” said the Great Sage, “and imprisoned me beneath this mountain. For over five hundred years already I have not been able to move. I implore the Bodhisattva to show a little mercy and rescue old Monkey!”
1.2. Warrior, not Mage
Wukong is a warrior-type fighter rather than a mage/wizard-type. This means that he depends heavily on his own abilities of physical combat and real skills. In fact, most of the villains on the journey could not withstand a duel of real skills with Monkey, and had to resort to their spells and magical treasures. A few examples of magical treasures are: Single-Horned Bovine’s hoop [src. 4], Silver Horn’s gourd [src. 5], Yellow Brows’ cymbals [src. 6], etc.; magic spells include: Zhenyuan’s sleeve [src. 7], Many Eyes’ golden beams [src. 8] and Red Boy’s fire [src.9]. With the use of these magic items against him, Monkey couldn’t do anything and was forced to retreat or be defeated, giving the impression that the demons were stronger.
1.3. Tired and Malnourished
The pilgrims don’t actually encounter villains consecutively. Demons always appear after they have travelled a long time, after a few thousand li . Therefore, Wukong’s energy would be drained from the exhaustion of travelling [src. 10]. In addition to tiredness, the supply conditions on the journey for the pilgrims were very poor, which caused Monkey, as well as the rest of the pilgrims, to be malnourished, not up to the health standard they should be at.
2. Why Monkey might be Holding Back
This second category explores reasons why Wukong chooses to not do his best when fighting monsters or rescuing Tripitaka. These reasons consider Monkey’s powers to remain the same as before his imprisonment, suggesting why he decides to not display 100% of his true abilities.
2.1. Understanding his Duty
For Tripitaka to successfully complete his pilgrimage, he must undergo a number of ordeals equivalent to the sacred number of nine nines (i.e. eighty-one). Wukong understands that reaching Spirit Mountain is not the sole task of the journey, the other part being enduring the 81 Tribulations. His knowledge of this is evident during the final ordeal at Heavenly-Reaching River (tongtian he, 通天河) [src. 11].
He [Monkey] knew, however, that the Tang Monk had not yet perfected the sacred number of nine times nine. That one remaining ordeal made it necessary for them to be detained at the spot [the west bank of Heaven-Reaching River].
By understanding that his role is to simply escort his master to endure these ordeals, Monkey would sometimes have to let down his strength, to allow Tripitaka to suffer slightly, as a contribution to accomplish the 81 Tribulations. This includes lowering his guard to allow Tripitaka to be deliberately captured [src. 12], agree to marriages [src. 13], be poisoned [src. 14], etc., all of which are done with safety assured (see 3.2) and are, in a sense, beneficial. If this was not done, then the sutras would not have been possible to collect.
2.2. Tripitaka's Safety is Guaranteed
A few courageous military officials led the captains and guards to rush forward and began hacking away [at Tripitaka, transformed into a tiger] with their weapons. If it had not been for the fact that the Tang Monk this time was not yet fated to die, even twenty monks would have been reduced to minced meat. Fortunately, he had at this time the secret protection of Light and Darkness, the Guardians, the Sentinels, and the Protectors of the Faith in the air. For that reason, the weapons of those people could not harm him.
2.3. Fear of the Headband
Monkey was afraid that if he became too violent in fights, and accidentally kills innocent people or too many people, his master would recite the Tightening-Headband Mantra (jingu zhou, 紧箍咒), which would bring his head great pain [src. 18]. This is why he wouldn’t display the full power of his abilities, and prefers to ask for reinforcements from Heaven instead, allowing to shift the blame for killing onto the gods. This also doesn’t require him to exert so much effort in strife.
3. The Main Reason
Wukong's struggles are mainly written so, as a plot device contributing to the overall story. The most enjoyable part of Journey to the West is the actual journey, where the pilgrims encounter various challenges that require skill and intelligence to endure. Without this part of the novel, there would be no Journey to the West, hypothetically with Monkey taking out every monster in one shot, or the pilgrims reaching Spirit Mountain without delay. Wukong is made to be as strong as the story needs him to be. Therefore, author Wu Cheng’en has a purpose to write Wukong this way to struggle in situations, so that his journey to enlightenment could be interesting for our amusement.
4. One False Belief
Contrary to the popular theory of many, the Tightening Headband that Sun Wukong wears does not, nor has it ever been mentioned in the novel to, constrict his powers. All it does is to keep the Monkey King under control because of his violent and immoral behaviour. The headband has no effect at all on Monkey’s performance, especially not restricting what abilities he can or cannot use.
4.1. A Reference of this Misbelief
One notable reference of this myth appears in the animated film Monkey King: Hero is Back (2015). The metal chain cuff that shackles Monkey’s right wrist acts as a substitute for the Tightening Headband from the novel. This constrictive shackle serves as a painful reminder for Monkey of his unruly nature which caused his imprisonment [src. 19], as well as a magical tool to restrict his abilities; it also cannot be removed or destroyed [src. 20]. Because of the shackle, Monkey is not able to access his divine staff or even fly [src. 21], making him sort of like a mortal hero of the story, slightly challenging the traditional Chinese fantasy norms.
Similar to the novelistic headband, the Monkey King’s constrictive shackle from the animated film naturally vanishes after he has completed his mission of saving the little boy’s village from monsters, in contrast with Monkey completing his task of escorting the Tang monk to the West.
There are quite a range of reasons why Sun Wukong doesn’t seem to perform to his full potential on his pilgrimage. These reasons explain both why Monkey cannot beat the demons and why he chooses to not display his full abilities. In the end, the main reason is that the author chose to write him this way to create a more engaging story. Sun Wukong really isn’t that much weaker after his imprisonment, there are simply other factors considered to explain this topic.
 over 600 years
Although the novel and all media states that Wukong was crushed under the mountain for 500 years, the novel actually mentions the years which Monkey was imprisoned and freed. The novel says that Monkey was imprisoned "when Wang Mang usurped the Han throne" (i.e. 8 AD), and he was freed by Tripitaka "in the thirteenth year of Zhenguan" (i.e. 640). This equates to a total of 632 years.
 li (里)
A li is a unit of measurement for great distances in traditional China. One li from the Ming dynasty, when Journey to the West was written, is equivalent to 576 metres (630 yards) in modern day.
 save him on three ocassions
There were three separate occasions where Monkey depended his life on these guardian deities:
These guardians disguised themselves as family of an old man and farmers, who provided Monkey and Pigsy with shelter one night. They went on to Monkey's eyes, after he was blinded by the Samadhi Wind of the Yellow-Wind Monster (huangfeng guai, 黄风怪).
When Monkey was crushed under three mountains by Great King Silver Horn (银角大王, yinjiao dawang), these guardians informed and instructed local mountain deities to help Wukong remove the mountains.
Monkey was trapped inside the Yellow-Browed Monster's (黄眉怪, huangmei guai) golden cymbals, where he is to be melted. It was these guardian deities who travelled up to Heaven to invite the 28 Constellations to save the Great Sage.
"Why does Sun Wukong seem so weak fighting demons on the journey even though he stormed the Heavens 500 years ago?"