In our previous work, the surface thermal distortion and far-field beam quality of 960-line spectral beam combining grating, which is irradiated by laser with different power densities, are studied by experiment and theory. The conclusion is that the thermal expansion of the substrate is the main cause of the distortion of the grating surface and the decrease of the beam quality. However, the improvement of grating surface heat deposition and the far-field beam quality are not considered. In this paper, the surface temperature, thermal distortion of grating and far-field beam quality at different irradiation power densities are analyzed with the improved model. The influences of the substrate thickness on the temperature, thermal distortion and far-field beam quality of the spectral beam combining grating are also calculated and analyzed. The conclusion is that the increase of substrate thickness can improve the power tolerance of beam combining gratings and the far-field beam quality of diffraction spots.