ing at the problem that the amplification of spontaneous emission (ASE) of high-power and large-diameter Nd∶YAG slab amplifiers a simplified three-dimensional ASE calculation model is proposed based on geometric optical ray tracing method, which corrects the diffuse reflection of the fluorescence side and the calculation error introduced by the temperature effect of the stimulated emission cross section. Through numerical simulation, the contribution weight of fluorescence reflection of each surface of large-diameter slab (size 150.2 mm×40.0 mm×2.5 mm) to the ASE effect is determined, and the gain of small signal under different pump powers are calculated. The effect of the ASE on the slab amplification performance is analyzed through the slab amplification. Experimental results and numerical calculations are compared, and the validity of the calculation model is verified.