At present, in most high-power laser drivers the phase modulation is used to suppress the effect of stimulated Brillouin scattering and meet the requirement for target surface light intensity uniformity. However, the modulated pulses show intense FM-to-AM effect after transmitted through a complex laser system. Analyzing the source of this effect is essential to the improvement of the output performance of whole laser system. In this paper, the theoretical analysis and the numerical simulation of the FM-to-AM conversion of the optical flat plates are investigated, which has been ignored by many researchers. The simulation results show that the more the laser passes through the optical flat plates, the deeper the modulation will be (for example, the modulation depth will be as high as 22.2% when transmitting ten times), and it follows a linearly increasing trend. What's more, the different flat optical elements have different spectral filtering effects. When the central wavelength of the front-end is chosen to be 1054 nm, the spectral filtering effect is very weak. To our knowledge, the reduction of the FM-to-AM conversion by changing the central wavelength is presented for the first time.