Abstract: The surface reflectivity change process of the 45# steel irradiated by repetitive pulse laser in atmosphere is studied by the measurement of the reflectivity and the metallographical and surface energy spectrum analysis of the recovered samples. The physiochemical mechanism of the processes of the 45# steel interacting with the laser in the atmosphere can be obtained by comparing the theory result with the experiment result.It can explain the change of the surface reflectance taking into account the temperature rise of the surface and the formation of the surface oxidant simultaneously.