Abstract: FFT algorithm and Rung Kutta method were adopted to investigate the frequency doubling process of the ultrashort laser pulse in KDP crystal. The effects of group velocity mismatch and dispersion, third-order nonlinear phase modulation on pulse shape, spectrum distribution and conversion efficiency of second-harmonic pulse were discussed. The results indicate that the modulation effect of the third-order nonlinear phase may distort the pulse shape of the frequency doubling, broaden the second-harmonic spectrum obviously and reduce the conversion efficiency. The third-order nonlinear phase modulation becomes the dominant factor to influence the process of the frequency doubling when the fundamental intensity of incident luminous power is over 100GW/cm2.