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The modulation of incident coherent beams is limited much less than 1 to linearize the equations easily in the study of the photorefractive effect with band transport model. The linearization can get good precision if the light modulation is small. However, in most cases the modulation approximates to 1 and the linear theory is not available any more. A new solution based on FlexPDE is put forward to solve the problem at large modulation depth. The photorefractive equations are rigorously solved based on the algorithm of finite element method. The forming of holographic grating in paraelectric KTN crystals at large modulation depth is researched. The distributions and time evolvement curves of space charge filed, light excitated electrons, ionized donors, and crystal refractive index are shown. Calculation indicates that the FlexPDE can reduce the difficulty of solving the photorefractive equations, and display the forming process with visualization of holographic grating dynamically on real time.