A theoretical model of quasi-three-level laser system is developed, in which both the thermally induced depolarization loss and the effect of energy-transfer upconversion are taken into account. Based on the theoretical investigation of the in-fl uences of output transmission and incident pump power on thermally induced depolarization loss, the output performance of 946 nm linearly polarized Nd:YAG laser is experimentally studied. By optimizing the transmission of output coupler, a 946 nm linearly polarized continuous-wave single-transverse-mode laser with an output power of 4.2 W and an optical–optical conversion efficiency of 16.8％is obtained, and the measured beam quality factors are M2x =1.13 and M2y =1.21. The theoretical prediction is in good agreement with the experimental result.