In order to quantitatively study the damage process of gallium arsenide(GaAs) wafer under infrared laser irradiation, a 1080 nm fiber laser was used as the light source. The scattered laser beam from both front surface and back surface of the wafer was collected. Then the damage process can be monitored in real time according to the scattering signal. A finite element model was established to study the evolution of the temperature field and scattering signal. The three regions of scattering signal corresponded to extrinsic absorption region, intrinsic absorption region and surface damage region, respectively. When the laser power density was 1.8 kW/cm2 and the irradiation time was 193 ms, slip lines can be observed on the initial damaged surface. The element content of the damage center was analyzed, and the oxygen element content increased greatly. These results indicated that thermal stress and oxidation reaction were the main damage mechanisms of GaAs. This study can provide theoretical and experimental basis for further research on temperature rise, thermal stress and ablation of GaAs wafer during laser irradiation.