Tuoba Si was born in 392 after his father Tuoba Gui had founded Northern Wei in 386 but before he had conquered most of rival Later Yan's territory and claimed imperial title in 399. His mother was Tuoba Gui's favorite consort, Consort Liu, the daughter of the Xiongnu chief Liu Toujuan (劉頭眷). He was born at the capital Yunzhong (雲中, in modern Hohhot, Inner Mongolia.) He was Tuoba Gui's oldest son, and his father was said to be so pleased by this late arrival of a son that he declared a general pardon. (Based on Northern Wei's official history, Tuoba Gui would only be 21 at this point; this might be further evidence corroborating the alternative version of his life history.)