Based on the grating diffraction theory, the duty cycle and refractive index change of the periodically poled microstructure (PPMS) crystal are studied theoretically by light diffraction efficiency. The duty cycle and refractive index change of PPMS LiNbO3∶MgO and PPMS LiNbO3 were investigated using a laser of wavelength 532nm and maximal power 80mW, 　respectively. The refractive index change of PPMS LiNbO3∶MgO is larger than that of PPMS LiNbO3.