In order to obtain better beam quality output, the gain distribution uniformity in diode side-pumped laser is studied. The gain field model of LD single-side pumped Nd∶YAG laser is established by using simulation software through theoretical analysis of the gain distribution in the working substance. In the simulation, the uniformity of gain field and absorbed light field of LD single-side pumped Nd∶YAG laser under different pumping power is compared. The result shows that the distribution uniformity of gain field and absorbed light field under different pumping power is not consistent, which is not a simple and intuitive equivalent linear relationship. Theoretical reference is provided for the high beam quality and high efficiency laser output of out-of-the-box LD side-pumped Nd∶YAG solid-state laser in high and low temperature environment.