Abstract: In order to obtain certain laser pulse with millijoule(mJ) order energy and nanosecond(ns) order pulse width after amplifying the pulsed seed laser, and to find the impact of repetition rate and simulated Raman scattering on laser pulse output, the dynamic model of a three-class ytterbium-doped double-clad fiber pulsed amplifier was established on the base of master oscillator power amplifier.Then the pulse energy,peak power,average power,pulse width and stimulated Raman scattering effect,arose from an amplifier of the pulsed seed laser with 10nJ energy and 100ns pulse width at different pulse repetition frequency,were simulated.The simulation results show that output laser energy and waveform are so slightly affected by the frequency that its effect can be ignored when the pulse repetition frequency is less than 200Hz,and that the Raman effect almost has little impact on the output property of each class amplifier if suitable parameters are selected.According to the numerical analysis,mJ order energy can be obtained from a three-class fiber amplifier with suitable parameters.