Aming at the coherent superposition of laser pulse reflected from the cavity mirror in multi-pass amplification process, a physical model for describing the amplification of coherent superstition of the rising part and the falling part of the pulse in a gain medium is established. According to theoretical analysis, standing wave is formed by the coherent superposition of the rising part and falling part of the pulse. The inversion population density shows up the effect of hole-burning by the standing wave, and the gain and pulse shape are affected. Based on this physical model, three conditions, i.e., no superposition, coherent superposition, and incoherent superposition are compared. This study will provide the instructions for high power solid laser design.