Using two beam-coupling equations to describe space-charge field, we investigated the inclination and the bent of the equiphase surface, which would cause two kinds of wave-front distortion of phase conjugate wave in four- wave mixing (FWM). The qualitative method destribing the distortion was obtained. Through numerical calculation, the effects of varied experimental parameters were presented. The length of the crystal and the applied electric field, which were related to the diffraction efficiency and the resolving power were also discussed.