To address the dual requirements of gain and stability in high-power fiber laser system, a high-gain and high-stability fiber amplifier is designed. The optical path employs single-fiber double-pass dual-stage amplification technology combined with an all-polarization-maintaining fiber structure to improve gain medium utilization. Additionally, a circuit system based on automatic power control and closed-loop feedback is developed, achieving dynamic regulation through a peak sampling circuit and precision pump driving circuit. Experimental results show that the system achieves a maximum gain of 28.25 dB, with closed-loop control reducing output energy fluctuations from 2.21% to 0.42%, improving stability by 81%. Moreover, it reduces the required number of pump lasers while delivering a pulse energy output of 70.2 nJ at a pump current of 260 mA.