In order to improve the output energy stability of fiber front-end system, an power automatic control optical fiber amplifier is designed with closed-loop feedback. The amplifier output signal splitting beem is used as a feedback signal. After photoelecrtic conversion and sampling by the high-speed peak hold circuit, the proportional integral differential(PID) algorithm is used to achieve an accurate control of the amplifier pump current. The experimental results show that under the condition that the energy of the injected large light pulse fluctuates, the P-V value of the fiber amplifier output energy is less than 8% and the root mean square(RMS) value is less than 1% in two hours. The stability of the output energy has been effectively improved.