Emperor Xizong of Tang
It is not known when Li Sijian—likely known as Tuoba Sijian at his birth—was born. His family was of Dangxiang stock, of the Pingxia () branch. His older brother Tuoba Sigong became a warlord during the reign of Emperor Xizong of Tang and, for his role in aiding the Tang Dynasty imperial government against the major agrarian rebel Huang Chao's state of Qi, was given the title of military governor of Dingnan Circuit, and was also given the imperial clan surname of Li. Li Sijian would have taken the surname of Li at the same time as well.