Because of interface imperfections such as interfacial roughness and diffuseness, the practical reflectivity of soft X-ray multilayer film is lower than the calculated one. Stearn scattering method describes the scattering of single non-ideal interface and is applicable for soft X-ray short wavelength region. Its mathematical model is used to describe the multilayer interface roughness, and the least square curve fitting, which is based on the test of fit, is used to fit the experimental reflectivity of Mo/Si multiplayer film by using synchrotron radiation, and good fitting results are obtained. Meanwhile the multilayer period thickness, the ratio of thickness, the interface width and the instrumental spectral resolution are analyzed. The parameters of multilayer such as roughness, diffuseness, period thickness and so on, impact of instrument′s spectral resolution on the reflectivity of multilayer were determined, and at the same time the fitting for the analysis of the structural parameters of multilayers is essential in the future fabrication of multilayers.