Cai Fu is a fictional character in Water Margin, one of the Four Great Classical Novels in Chinese literature. Nicknamed "Iron Arm", he ranks 94th among the 108 Stars of Destiny and 58th among the 72 Earthly Fiends.
Living in Daming Prefecture (大名府; present-day Daming County, Hebei), Cai Fu is the chief warden of the city's prison and also its chief executioner assisted by his younger brother Cai Qing. Cai Fu is nicknamed "Iron Arm" because he can chop off a prisoner's head with just one swing of the sword saving the person unnecessary pain.
Lu Junyi, a squire of Daming, is arrested and jailed after his housekeeper Li Gu, who is having an affair with Lu's wife, reports him to the authorities that he has ties with the bandits of Liangshan. Lu has earlier been lured to Liangshan, which failed to persuade him to join them.
Li Gu bribes Cai Fu, the warden in charge of Lu Junyi in prison, to murder the squire. However, Liangshan's Chai Jin also visits Cai Fu and offers him an even larger sum of money to ensure Lu is safe. Not wanting to antagonise either, Cao Fu is in a bind. Cai Qing advises him to bribe officials to exile Lu to a faraway place, thereby washing his hands of the case.
However, Li Gu bribes the guards escorting Lu Junyi to Shamen Island (沙門島; present-day Changdao County, Shandong) to murder him along the way. Lu's servant Yan Qing kills the escorts and saves Lu when they are about to finish him off in the woods. But Lu, who is weak from days of torture, is captured again at an inn when Yan goes looking for food. This time, he is immediately sentenced to death. Just when Cai Fu is about to swing his sabre to chop off Lu's head at a public execution ground, the brothers are gladly stunned by Shi Xiu of Liangshan, who storms towards them in a lone attempt to save the squire. The brothers, who do not want to be in the black book of Liangshan, subtly loosen the ropes on Lu and allow Shi Xiu to drag him away. But the two are overwhelmed by the soldiers and captured. Grand Secretary Liang Shijie, the governor of Daming, decides to keep them alive as bargaining chips should Liangshan mount an attack. The Cai brothers take good care of Lu and Shi in prison.
Liangshan could not break into Daming despite winning a few military battles. After a respite that caused Daming to become less vigilant, Wu Yong, Liangshan's chief strategist, sends a number of chieftains to infiltrate Daming on the night of the Lantern Festival. When they unleash havoc in the city cued by a fire set by Shi Qian, Chai Jin visits the prison to coerce the Cai brothers to free Lu Junyi and Shi Xiu. Knowing they have no choice, the Cais release the two prisoners and follow the outlaws back to Liangshan.
Cai Fu is appointed Lianghsan's chief executioner after the 108 Stars of Destiny came together in what is called the Grand Assembly. He participates in the campaigns against the Liao invaders and rebel forces in Song territory following amnesty from Emperor Huizong for Liangshan.
In the battle of Qingxi County (清溪縣; present-day Chun'an County, Zhejiang) in the campaign against the rebel leader Fang La, Cai Fu is critically injured. He later dies from the wounds.