According to a story in his biography in Book of Jin, while Tan was still his father's heir, he wore a golden bell that one day shrivelled up like millet.
A fortune teller believe that the gold represented the prosperity of the Jin dynasty, and was a good omen for Tan who would one day ascend the imperial throne.
Real power in Luoyang was held by Shangguan Si, and as he became increasingly cruel and violent, the ministers, Zhou Fu and Man Fen attempted to depose him but failed.
One night, Sima Tan launched a surprise attack on Shangguan Si and Miao Yun, causing the two men to flee the city.
Despite the warm welcome, Zhang Fang soon confirmed his deposition as crown prince and replaced him with Emperor Hui's half-brother, Sima Chi in February 305.
At the end of 307, there was a plot to establish Sima Tan as crown prince by Lü Yong (呂雍), Chen Yan (陳顏) and others.
However, Sima Yue discovered the plot, and now convinced that Tan would only cause further trouble, he forged an edict imprisoning him at Jinyong Fortress (金墉城) near Luoyang.