针对脉冲周期在ms量级、脉宽为ns量级的高功率激光系统输出的短脉冲串，根据其周期性特征及放大过程的特点，在忽略损耗系数和自发辐射的情况下，将1个周期近似分为泵浦和放大2个阶段。运用端面泵浦瞬态速率方程组，导出了2阶段变换时刻所对应的上能级粒子数之间的半解析关系式。据此，在模拟光纤长度对增益影响的基础上，研究了2阶段腔内平均上能级粒子数密度随时间的周期性变化关系，定量分析了脉冲前沿消耗增益对脉冲后沿形状的影响。结果表明：光脉冲前沿增益可达29.22 dB，其后沿增益低至0.82 dB，光脉冲后沿波形必然存在畸变，验证了所得结论的合理性。

For a high power shortpulse train with a period of microsecond level, and a pulse width of nanosecond level, the amplification process of the pulse train by a fiber amplifier in one period is approximately divided into a pumping stage and an amplification stage, considering the periodic behavior of the pulse train, and neglecting the loss coefficient and spontaneous emission. By using the endpumped dynamic rate equations, the semianalytical relation related to the average value of upperlevel population in the two stages, has been established. Then the impact of fiber length on the gain is simulated, and the periodic variation of average value of the upperlevel population is discussed.Pulse deformation of the train due to the depletion of gain by the pulse front is quantitatively analyzed. The results show that the gain of the pulse front can be about 29.22 dB, while that of the pulse trail is low to 0.82 dB. Thus the deformation of the pulse trail is bound to exist.