In order to study the active spontaneous emission of erbium-doped fiber pumped by arbitrary waveform pulse with breaking the limitation of pump waveform before. By dividing the time into tiny periods, the expressions for level population and the average power of active spontaneous emission were obtained with rate equations. Simulations and experiments show that pump glitches have less effect on the active spontaneous emission just like having a “high frequency filter”;the waveforms of the pump light and active spontaneous emission light are alike when the pump power is large relatively, this research can be used to all-optical modulation for the optical fiber laser. The results are consistent with the theoretical analysis, which show the analysis is reliable.