For an all-fiber single-frequency amplifier,a passive transmitting fiber of the pump coupler may induce unwanted stimulated Brillouin scattering and affects the amplifier performance.To evaluate the impact,a rate-equation based model has been adopted to study SBS characteristics in the fiber amplifier with a transmitting fiber.The model allows independent inputs of many critical parameters,such as pump schemes,the fiber length and geometrical structures.The results of simulation provide a theoretical base for optimizing the design of high power single-frequency fiber amplifier.