In order to study Stimulated Brillouin Scattering(SBS) in Yb3+-doped double clad single-frequency fiber amplifier,thermal effects of the fiber amplifier were analyzed theoretically,based on the rate equations and heat transfer equations.The impacts of the fiber temperature,which can broaden the SBS gain profile and suppress SBS,on the amplifier performances were discussed.The relationships between the SBS and pump power,signal power,heat transfer coefficient were simulated.The results of simulation provide theoretical guide to optimum design of high power single-frequency fiber amplifier.