Strong applied electric field and large modulation have been widely applied in photorefractive effect research. Photorefractive band transport equations are rigorously solved based on the algorithm of general collocation software for partial differential equations (PDECOL). Temporal evolutions of space-charge field and charge carrier density in photorefractive crystal have been achieved under different light modulations, and effects of an applied electric field have also been considered. Material equations and coupled wave equations are jointly solved to get the coupling coefficient and phase shift of light pattern during the dynamic procedure of photorefractive grating formation. It is found that, under strong modulation, the intensity coupling and phase coupling between two recording beams are both markedly enhanced, while the analytic equations, which neglect the effects of strong applied electric field and large light modulation on the coupling of the phase of the space charge filed, are not valid any more. Numerical results also show that both the light pattern and index grating are bent and do not keep parallel.