Co/Sb multilayer shows the theoretical reflectance of 62% at a photoenergy of 528 eV and the grazing angle of 10.3°, but the measured reflectance is very low, which is considered to result from the low energy resolution of the incidence light. In this paper we have investigated the effect of the energy resolution of the incidence light on the measured reflectance, and show a new method how to calculate the reflectance of non-monochromatic light. The calculated data show that the reflectance of Co/Sb multilayer is very sensitive to the effect ofthe energy resolution of the incidence light. Generally, it is very low unless the energy resolution of the incidence light is high enough. Using our method, we give the fitted result of the reflectance spectrum. Our fitted result is consistent with the measured reflectance spectrum.