To describe an effective heat source caused by the reflectance variation under laser irradiation, a dynamic model of reflectance evolution consisting of a multi-layer oxide film growth model,a multi absorbing film on absorbing substrate model and a heat diffusion model is build. The oxide film consists of three layers of Fe2O3, Fe3O4 and FeO. The oxidation rates of Fe2O3 and Fe3O4 layers are represented by a linear rate law at initial stages and a parabolic rate law at later stages than 100 nm. The Fe3O4 layer and FeO layer form films from 250 ℃ and 570 ℃. The multiabsorbing film on absorbing substrate mode is used to calculate the reflectance of the multi-laryer oxide film and its temperature is computed by the thermal conductive equation, by which the time dependance of reflectance on temperature is obtained.Finally, a time dependence of reflectance and temperature are calculated. The total integrated scattering measurement device is established to measure the reflectance and temperature of the 45# steel irradiated by 1.06 μm cw laser with different powers in-situ. The numerical solutions are in agreement with the experimental data.