In this article, by using variational method, we study the existence of a positive ground state solution for the Schrodinger-Poisson system -Delta u + V(x)u + K(x)phi u = f(x, u), x is an element of R-3, -Delta phi = K(x)u(2), x is an element of R-3, where V (x), K (x) and f (x, u) are asymptotically periodic functions in x at infinity.