The Green function of wide-angle parabolic equation (WAPE) in boundless plane is constructed based on the split-step Fourier transform (SSFT) algorithm.A method for calculating the initial fields of WAPE by using the Green function is presented.Based on this method,wave propagation on sea surface by a Gaussian current source is calculated by using the WAPE.Results comparison between the WAPE,the two ray model of geometrical optics (GO) and the Green function itself testifies the correctness of the Green function.Accordingly,the groundwork of a new approach so-called assistant potential function method for calculating the PE is established.