신당서
It is not known when Song Shenxi was born, and, while his family was listed as "the Songs of Guangping" (廣平, in modern Handan, Hebei) in the table of the chancellors' family trees in the "New Book of Tang", Song's biography in the "New Book of Tang" indicated that his family's origin had been lost to history. It is known that his grandfather was named Song Su () and that his father was named Song Shuye ().