Rankin-Selberg L-function of general linear groups is an important example of automorphic L-functions and it has many applications in number theory and Langlands program. In this talk I will report our recent construction of integrals which represent product of Rankin-Selberg L-functions of $GL_l\times GL_m$ and $GL_l\times GL_n$ if $m+n<l$. This talk is based on a joint work with Pan Yan.