Abstract: In order to study the surface thermal decomposition damage of millisecond pulse lasers irradiating GaAs at 0.53 m wavelength, a two-dimensional axisymmetric heat conduction model was established. Considering the temperature dependences of the thermophysical parameters, the transient temperature fields in the material were simulated by using the finite element method. The distributions of transient temperature fields and variation law with time have been obtained, and the curve of the thermal decomposition damage thresholds on the material surface was also given. Numerical results show that, heat conduction affects the whole process of millisecond pulsed laser interaction and its damage mechanism is mainly thermal effects. The temperature of the center is the highest on the irradiated surface, where it could be damaged by thermal decomposition first. With the increasing of laser energy density, the time of thermal decomposition damages occurrence would be advancing.