In spectroscopic ellipsometric measurement, the backside reflection of the glass substrate has great influence on the tested results. Ellipsometric measurement and model calculation are carried out for the silicon nitride film deposited on glass substrate of flat panel displays. The coherent model of the baskside reflection, i.e. "air-substrate-air" is used to calculate and obtain the refractive index of the glass substrate which agrees well with the data provided by manufacturer. Tauc Lorentz model is used in data analysis for the silicon nitride film to discuss the effect of the interface layer between the film and substrate, the surface roughness on the optical constants and model fitting. The results suggest that the inclusion of the interface is necessary for improving the degree of fitting in the case of lattice mismatch between the film and substrate. The optical constants and film structure of the film system are presented.