The crystal′s thermal effects under different thermal boundary and pump structure were reported. In theory, based on operating characters of space laser, a thermal model satisfying crystal working state was set up. The temperature distribution of Nd：YAG crystal was simulated for diode laser with Gaussian beam distribution side pumped laser, and the influence of thermal boundary condition, pump manner and pump parameters on thermal distribution was analyzed. With assumption that the thermal lens of crystal was thin lens, the thermal focal length was calculated theoretically and measured experimentally. With five-side annular pump structure, pump power of 4 500 W and pump repetition frequency of 10 Hz, the thermal focal length of Nd：YAG was about 9.5 m. Experimental results are in basic agreement with the simulated results.