A universal and convenient matrix series method to solve the Raman Nath equation of normal ultrasonic light diffraction is presented. The results show that the diffraction efficiency for Bragg approach is only 97.5% at Q=4.1π, and that for asymmetry incidence, there is large error in previous Raman Nath approach. The formula of the optimum length of the acousto optics modulator is given. Increasing the ratio of acoustical frequency to optical frequency is a effective way to improve the diffraction efficiency.