He succeeded his father Li Gao (Prince Wuzhao) in 417 and aggressively tried to pursue campaigns against rival Northern Liang's prince Juqu Mengxun, but fell into a trap set by Juqu Mengxun in 420 and was killed in battle, bringing destruction to his state (although his brother Li Xun tried to hold out but was defeated soon as well).
In 419, with Li Xin's punishments and construction projects overburdening his people, his officials Zhang Xian (張顯) and Fan Cheng (氾稱) advised him to be more lenient and more thrifty, pointing out that he would otherwise not be able to defeat Juqu Mengxun.
Li Xin, believing wrongly that Juqu Mengxun's defenses were down, decided to attack Zhangye, against the advice of Song Yao and Zhang Tishun.
Princess Dowager Yin also spoke against it, pointing out that he had insufficient strength to conquer Northern Liang and warning him that a defeat would destroy his state.
His generals then advised him to quickly withdraw to Jiuquan, but Li Xin, stating that he had disobeyed his mother and would only be able to see her again after a victory, engaged Juqu Mengxun again, suffering an even greater defeat, and he was killed in battle.