Based on the heat conduction theory, a twodimensional axis symmetric transient physical model was established, of which GaAs was irradiated by Gauss laser. By solving the heat conduction equations using COMSOL Multiphysics analysis software, the curve of material surface temperature and the relationship between the temperature at the center of the light spot and material thickness were obtained, and then the relationship between threshold damage time of GaAs and the incident light power density was gained. The calculation results show that the laser induced damage is mostly decomposition damage in the areas where temperature exceeds decomposition temperature of material. The theoretical analyzing result is in accordance with the relative experimental result, so the laser irradiation effect model proves correct.