Li Chengqian was created crown prince in 626 at the age of eight (by East Asian reckoning), after his father became emperor on 4 September.
He lost favor in Emperor Taizong's eyes to a younger brother, Li Tai the Prince of Wei.
He was posthumously granted an imperial prince title during the reign of his grandnephew, Emperor Xuanzong after his grandson Li Shizhi became chancellor.
Parents Consorts and their respective issue(s)[8]: Li Chengqian was born in 619, not long after his grandfather Emperor Gaozu established Tang dynasty in 618.
Later that year, Emperor Taizong formally made Li Chengqian in charge of listening to legal appeals where the litigants were dissatisfied with the judgments made by the executive bureau of the government (尚書省, Shangshu Sheng), putting a layer of appeal between the executive bureau and the emperor himself.
Meanwhile, by 633, Li Chengqian was said to be beginning to favor frivolous games, and his staff members Yu Zhining, Du Zhenglun, and Kong Yingda often advised him to not do so.
Empress Zhangsun disallowed it, disfavoring pardons herself and knowing that Emperor Taizong also disapproved of Buddhism and Taoism.
Li Chengqian, meanwhile, was allowing his games to interfere with his studies, despite urgings of his advisor Zhang Xuansu (張玄素).
In 641, When Emperor Taizong visited the eastern capital Luoyang, Li Chengqian was in charge of Chang'an, assisted by the chancellor Gao Shilian (his granduncle).
It was said that Li Chengqian favored music, women, and hunting, and was wasteful in pursuing these things, but at the same time maintained a veneer of virtue to the outside world.
Li Chengqian had a young musician as his catamite, whom he nicknamed Chengxin (稱心, meaning, "satisfying my heart").
He had also favored Taoist monks Qin Ying (秦英) and Wei Lingfu (韋靈符), and he had them use magic—acts that were considered extremely disapproved among imperial subjects.
Eventually, he also invited the major general Hou Junji, whose son-in-law Helan Chushi (賀蘭楚石) was Li Chengqian's guard commander, into the plot, and considered overthrowing Emperor Taizong.
Emperor Taizong, in shock, convened the senior officials Zhangsun Wuji (Empress Zhangsun's brother), Fang Xuanling, Xiao Yu, and Li Shiji, as well as officials from the supreme court, the legislative bureau, and the examination bureau, to investigate, and the extent of the plot was revealed.