Both the interfacial roughness of multilayer and energy Monochromaticity of the incident light influence the practical reflectivity of multilayer. With convolution integral, a new formulation for describing the relation of soft X-ray reflectivity of multilayer with the different energy Monochromaticity of incident light is deduced. Using this formulation, the effect of different energy Monochromaticity and interfacial roughness on the reflectivity spectrum of Mo/Si multilayer is analyzed respectively. The results of theoretical analysis show that the low energy resolution reduces sharply the peak reflectivity of Mo/Si multilayer but increases the background intensity, which leads to the increase of the full width of half maximum of reflectivity spectrum and the distortion of reflectivity spectrum. However, the interfacial roughness of multilayer only decreases the value of the reflectivity and does not change the shape of reflectivity spectrum and full width of half maximum of reflectivity spectrum. These results illustrate that the influence of energy Monochromaticity of incident light on the reflectance of multilayers is different from that of the interfacial roughness.