Abstract: Most  of  the Emperor’s  marriage were arranged  for political reasons,including  the Manchu  and  Mongolia  unity by  marriage.But  in the Post-Jin  epochs ,the   unity by  marriage  of royal families  between Manchu  and  the Kerqin  Mongolian  peoples occurred  forty  times,for  more  than that between Manchu  and  other  Mongolian    peoples.Therefore  many social  factors beyond  the political were included in  the unity  by  marriage.This  paper  examined  the circumstances   in  which  the  Manchu  and Mongolian  peoples lived,and discussed  the main caused  of the unity by  marriage，the social functions  of  that  and the interrelations  between  the  two  peoples.