During the reign of Emperor Cheng 漢成帝 (r. 33-7 BCE) he was appointed gentleman of the interior (langzhong 郎中) and became Director of the Music Bureau (yuefu ling 樂府令). Under Emperor Ai 漢哀帝 (r. 7-1 BCE) he studied astronomy and compiled the short treatise Huntianshuo 渾天說.
When Wang Mang 王莽 (r. 8-23 CE) usurped the throne of the Han dynasty, Huan Tan did not praise the new emperor and therefore occupied only a small position in the music section of the court. Under Emperor Guangwu 漢光武帝 (r. 25-57 CE) he was recommended by Song Hong 宋弘 and promoted to court gentleman for consultation (yilang 議郎) and palace steward (jishizhong 給事中).
As a philosopher, Huan Tan preferred to find out the great meaning of the Confucian texts instead of interpreting each and every single sentence, as it was common during his time. He also did not engage in the strife between the old-text and the new-text school and refused the popular interpretation of natural disasters as an expression of the will of Heaven over the ruler and the country.
Yet Emperor Guangwu loved the apocryphal texts (chenwei 讖緯) presented to him because such could support his claim for the throne. Huan Tan criticized such writings because he did not see such superstitious beliefs in accordance with Confucius' teachings of benevolence and righteousness to be expressed by the ruler. When Emperor Guangwu wanted to erect an astronomical terrace, the Lingtai 靈臺, he wished to employ geomancy to determine an auspicious location, and again, Huan Tan criticized this as not in accordance with the Confucian texts. Highly enraged, the emperor wanted to execute Huan Tan, but then pardoned him and sent him to Liu'an 六安 into exile. Huan died on the way to this remote town in the Yangtze valley.
Huan Tan was of the opinion that there is a soul or spirit (jingshen 精神) in the body that is supported by good deportment and so will never extinguish. Like the flame of a candle can not leave the candle, the spirit can not leave the body. Huan Tan has written the book Xinlun 新論 that is only preserved in fragments. Parts of his writings are assembled in the collection Quanhanwen 全漢文 published by the Qing period 清 (1644-1911) scholar Yan Kejun 嚴可均.