He hailed from Pingling 平陵 (near modern Xianyang 咸陽, Shaanxi) and had been educated by the Confucian scholar Zhang Shanfu 張山拊 who taught him the Classic Shangshu 尚書 "Book of Documents" in the version of Xiahou Junior 小夏侯 (Xiahou Jian 夏侯建), as well as by Zhang Ru 張孺 and Zheng Kuanzhong 鄭寬中.
Li Xun had a special interest in the theory of the Four Agents as explained in the chapter Hongfan 洪範 of the Shangshu, in which natural phenomena are correlated to political performance. For this expertise, Chamberlain for the National Treasury (dasinong 大司農) Wang Gen 王根 consulted Li Xun.
When Emperor Ai 漢哀帝 (r. 7-1 BCE) mounted the throne, he was granted regular access to the imperial palace and was consulted each time a strange event had occurred. Li Xun was finally appointed Commandant of cavalry (jiduwei 騎都尉) and was instructed to inspect the canals, dams and dykes in the empire.
In 5 BCE, Li recommended Xia Heliang 夏賀良 to the emperor who suggested changing the reign motto and to adopt a new title of the emperor namely Chensheng Liu taiping huangdi 陳聖劉太平皇帝 "Emperor of Great Peace, stabilizing the sacred family Liu". Yet his title was soon abandoned, Xia Heliang was executed, and Li Xun escaped to the commandery of Dunhuang 敦煌 in the far west.