Sinusoidal phase modulated pulse could provide broad bandwidth and high modulation frequency for spectral dispersion method in high power laser driver system. Phase modulation could be converted to intensity modulation due to dispersion during the pulse propagation in the fiber,and the intensity modulation enhances the nonlinear effect in the large-mode-area double-clad fiber amplifier and make the spectral controlling more difficult. 2 GHz and 1425 GHz phase modulation pulses propagated in fibers with different lengths were measured, and based on the dispersion model in the fiber, theoretical analysis shows good similarity between simulation and experimental results for the 2 GHz phase modulation pulse.