Abstract: To study the reflectivity character of alloy steel under CW-laser irradiation, the integrating sphere reflectometer was setup to measure the reflectance and temperature of 45# steel in -situ. Experimental results show that the reflectance dropped dramatically first, and then fluctuate periodically. The amplitude decreases and the period increases with time. With the increase of laser power, the temperature correspond to the begin point of decline and minimum point increases too. The number of maximum value is increased, the maximum value is increased one by one instead of decreased. A multilayer oxide films affection theory model was built. Combining with the model and oxide film growth mechanism of 45# steel, the reflectance was calculated at different oxide film structure and thickness ratio. The comparison between simulation and experimental results shows that the theory is reasonable. The study indicates that the multi-beam interference effects and absorb effects induced by oxide film growth are main reason of the reflectance evolution. In three layers of FeO-Fe3O4-Fe2O3, the absorbing Fe3O4 film is the dominant layer, the outer film Fe2O3 can cause multi-beam interference, and the inner FeO film hardly has contribution to reflectance.