Abstract: In order to study the stimulated Brillouin scattering(SBS)effect on the pulse form of pulsed fiber laser amplifiers,a single frequency fiber laser amplifier system was built and its pulse waveform distortion was observed in the experiment.If continuing to improve pump power,a high power spike appeared at the distorted location,which was induced by the second-order SBS effect in a fiber.To verify this conclusion,the coupled equations contains the second-order Stokes wave were built and simulated and the results showed that the high peak power spike was the second-order Stokes wave of SBS effect.