Based on the nonlinear,time-dependent radiation transfer equations,which describe the amplification process of a pulse and account for the effect of the radiation on the medium and vice versa,the power characteristics of the output pulses of a Yb-doped fiber amplifier are investigated under different input pulse shapes.Numerical results show that,the power gain of the pulse traversing the amplifier is different for different shapes when the average energy of the input pulse is the same.This difference is larger in the leading part of the pulse,which leads both the gain value and the shift value of the peak power for the output pulses to be dependent on the input pulse shape.Especially,when the input pulse energy is relatively large,the gain of the peak power for different pulses is obviously different; with Super Gaussian pulse the largest followed by Gaussian pulse and Hyperbolic-Secant pulse and Lorentzian pulse the smallest.