Abstract: Temperature dependence of reflectivity of 45# steel at the wavelength of 1.319m was measured by using an integrating sphere system. The change of the reflectivity was explained by analyzing the surface energy spectrum of steel plates, which was heated. Reflectivity didnt change remarkably in the range of room temperature to 325℃, and it decreased from 0.64 at 325℃ to 0.15 at 395℃. As the temperature kept on increasing, the reflectivity didnt change obviously. In the cooling process, reflectivity held as a nearly constant value, which was the same as the value at the highest temperature. The change of reflectivity is mainly due to the oxidation of surface. As the temperature rises, oxidation reaction becomes severe and the atom percentage of oxygen in the surface increases, so the reflectivity decreases.