Lidar has been widely used in both military and civil applications. Its received optical signal undergoes considerable loss and disturbance from background noise, which limits its performance especially in bad weather or air conditions. A high-gain and frequency-selective amplifier for weak optical signal based on stimulated Brillouin scattering in single mode fiber is proposed, which is an excellent candidate for the signal enhancement in lidar system. The characteristics of the amplifier were studied numerically and experimentally. In experiment, a 430-nW (peak power) pulsed signal was amplified by 70 dB with a signal-noise ratio of 14 dB, which was in good agreement with theoretical result.