The gear- parallelogram mechanism（ GPM） is a kind of geared linkage combinative mechanism with special motion performance. The composition rules of GPM are introduced,and the transmission property of outer teething type and idle gear type of GPM are analyzed. Meanwhile,according to the principle of the variation of the isomorphism,the evolution and mutation of internal teething type is studied. By the comparison of internal and external meshing combined mechanism,the method of changing different mechanism transmission ratio is summarized and the type of internal teething and internal parallel motion of GPM is chosen,which has large transmission ratio,high mechanical efficiency,small size,light weight,compact structure and better smoothness of moving. Some theoretical basis for design new combined mechanism about reducer and speed increaser is provided.