Volume bragg grating is the important element in spectral beam combining technique. However the thermal-induced distortion and temperature gradient make strong impact on the grating’s performance. In this paper, a model of five 3kW off resonator laser beams combining system is analyzed through Comsol. The z-displacement and temperature distributions with different boundary conditions have been shown. The results demonstrate that Comsol is a practical and useful tool to analyze the thermal problem in the gratings.