In the study of laser damage mechanism, the change rule of incident laser reflectivity by materials versus the irradiated surface temperature is always concerned. Under the irradiation of a 915 nm laser, the change of the 3.8 μm probe laser reflectivity by 915 nm laser irradiated 45# steels is measured by the reflectivity measurement system based on the integrating sphere method. The temperature change of the irradiated regions is determined. The results show that the steep decrease of reflectivity is caused by the rapid oxidation of specimen surfaces at high temperature, and the slow variation of reflectivity with temperature is the result of the variation of conductivity with the temperature change.