During the study on the multi-pass amplification of nanosecond light pulse, analytical results of Lowdermilk et al just care about two critical situations, namely, gain full-recovery and gain non-recovery. In fact, the gain recovery of amplifier during the pulse interval is most likely between those two recovery situations. In this paper, the equations about relationship between pulse fluence and population in the upper and lower energy level have been built up. Moreover, sequential relationship was deduced between time intervals during which light pulse interacts with population density in the upper and lower energy levels. Thus, the problem of gain non-recovery has been solved Moreover, the results calculated by this model were compared with those of Lowdermilk analysis model and multi-pass amplification experiment It was also analyzed that how the decay effect influenced on the characteristics of energy amplification· And relatively good and reliable results were achieved.