By treating photorefractive spatial soliton as rays in geometric optics, the action of the diffusion effect on propagation of PR spatial solitons is analyzed. Based on similarity between ray picture and classical mechanics picture, ray Hamiltonian equations of motion in a diffusion potential created by the induced change in the refractive index are resolved exactly by integrating. Results indicate that the center of the optical spatial solitons moves on a parabolic trajectory and the spatial frequency transverse component shifts linearly with propagation distance.