Output characteristics of Yb-doped double-cladding pulsed fiber amplifiers were discussed under conditions of low and high repetition rates using the time-dependent rate equations. Because there are several different initial conditions used in the finite-difference method with different repetition rates, some methods were presented to determine the initial conditions. The relationship of the peak powers of the high repetition rates with their equivalent continuous wave powers was also discussed when the signal light was in Gaussian shape. The change of the average output power with the pump power was obtained under the conditions of the pulsed signals with low repetition rates and high repetition rates.