Two wave coupling equations on anisotropic diffraction in photorefractive crystal is deri-ved according to coupling wave theory. Phase gratig diffraction results of a He-Ne laser beam (λ= 632.8 nm) passing through tungsten bronze crystals KNSBN doped respectively Ca, Ce and Co are given. The anisotropic diffraction patterns of these three kinds of photorefractive crystals is formally analyzed using these coupling formulae. It is concluded that the anisotro-pic diffraction ring pattern will present in one of the two kinds of diffraction (e→o or o→e diffraction) for different (positive or negtive) unixial crystal. Theory analysis and experi-mental results agree fair by well.