Cai Qing was the younger brother of Cai Fu, and served as an executioner and jailer in Damingfu, Beijing with his brother. He had large eyes and thick brows, was a man of strong character, and loved to stick a flower in his hat, thus earning him the nickname 'Flower' (一枝花).
Once, Lu Junyi was betrayed by his adulterous wife and housekeeper Li Gu and thrown into prison. Li Gu tried to bribe the Cai brothers to kill Lu Junyi, but the Cai brothers accepted a larger bribe from Chai Jin and agreed to look after Lu Junyi instead of killing him. After some discussion, the Cai brothers decided to help Lu Junyi so Lu was not sentenced to death and was only sentenced to exile at Shamen Island.