The stimulated Brillouin scattering(SBS)effect is easy to be observed in optical system because of the huge power of laser in ICF experiment. To avoid its damage,phase modulator is used to increase the threshold of the SBS effect. Simulation is used to discuss the situation of laser pulse after amplitude modulator,phase modulator and filter. Corresponding to the experimental result of 4.3 GHz(0.23 ns in time domain)fluctuation on the top of the pulse,it is obtained that lack of high frequency part of the laser pulse causes the fluctuation with the frequency of phase modulator,while the unbalanced sidebands make this effect easy to be observed. The influence of the shape,bandwidth and mismatch of the filter on the pulse fluctuation are also discussed. Furthermore,a way to suppress the high frequency fluctuation is proposed. When the filter detunes with the center wavelength of the laser,a Gaussian filter is better for small bandwidth,whereas ideal filter is better for bigger bandwidth situation