Sima Dan was born in 343, during the reign of his father Emperor Kang, by Empress Chu Suanzi, Emperor Kang's wife. He was his father's only son. When he was only one year old in 344, Emperor Kang grew seriously ill. His granduncles from his paternal grandmother's side, the key officials Yu Bing (**) and Yu Yi (**), wanted to support his granduncle, the son of his great-grandfather Emperor Yuan, Sima Yu the Prince of Kuaiji, as the new emperor, but Emperor Kang accepted the advice of another key official, He Chong (**), and decided to pass the throne to Sima Dan despite his young age.
Yu Bing
Yu Yi