Abstract: In this paper, we study the thermal effects of side-pump large energy solid state laser. We set up a numerical model of LDA side pump Nd:YAG laser rod. Using Ansys program, the finite element method is adopted to analyze the time-varying thermal effect for pulse pump solid-state laser. The steady temperature field and the transient temperature field distribution are simulated, and the changing rule of the distribution of temperature field in the medium is summarized. We calculate the thermal focal length caused by thermal gradient, stress birefringence, and the end effect of medium respectively. The research results show that the thermal effect of LDA side pump laser show time-varying characteristics, the center temperature in cross section of medium have a zigzag periodical change, and the frequency equals to LDA pump frequency. The steady-state distribution is influenced by LDA pump parameters greatly. The thermal lens effect is mainly caused by the temperature gradient in the medium. This study could provide a theoretical guidance for the design of thermal-stable laser resonator and compensation of thermal effect.