Based on the theory of broadband pulsed laser amplification in the homogeneously broadening medium, a numerical model for the temporal pulse shape propagating through the regenerative amplification is established by means of the short-time Fourier transform. Numerical simulation results show that decreasing the detuning between laser wavelength and the center wavelength of gain medium can decrease the FM-AM effect effectively. Increasing the energy of the seed laser pulse can also decrease the FM-AM effect and thus improve the laser beam quality.