In order to investigate the thermal stresses field of GaAs during the millisecond pulsed laser irradiation, a 3D numerical model has been established based on the thermoelastic theory with the finite element method (FEM). T he 12 slip systems can be obtained to describe the dynamic change process of resolved shear stress field of GaAs during the laser irradiation. The results show that the thermal slips are firstly activated below the spot area of laser when the thermal shear stresses of slip systems surpass the critical yield stress, and this comes out before the melting of GaAs surface. Brittle cracks occur in the initiation points offered by the thermal slips, and this can greatly reduce the photoelectric performance of GaAs. The calculated stresses distribution indicate that the fracture mostly occurs at the boundary of the laser spot and the center, which is consistent with the experimental result.