司马越
Sima Pi was born in 341, during the reign of his father Emperor Cheng of Jin, as Emperor Cheng's oldest son. His mother was Consort Zhou, who in 342 gave birth to his younger brother Sima Yi. In summer 342, Emperor Cheng grew ill. The common succession protocol, as He Chong () pointed out, would mean that his oldest son would succeed to the throne, but Emperor Cheng's uncle Yu Bing (), wanting a new emperor who would also be connected to his clan as well, persuaded Emperor Cheng to pass the throne to his younger brother Sima Yue the Prince of Langya, also a son of his sister Empress Yu Wenjun, under the reasoning that with the powerful rival Later Zhao to the north, the empire needed an adult emperor. Emperor Cheng agreed, and named Sima Yue as the heir, and Sima Yue succeeded to the throne as Emperor Kang when he died soon after. The one-year-old Sima Pi was instead created the Prince of Langya. He continued in that title after Emperor Kang's death in 344, as Emperor Kang chose to pass the throne to his infant son Sima Dan (as Emperor Mu). It is not known when he married his wife Wang Muzhi, only that she carried the title Princess of Langya.