The fitting model of multi-layer thin films in ellipsometry analysis was established. An unknown film was investigated and the physical structure model of substrate/film1/EMA/film2/srough was applied. Based on spectroscopic ellipsometry after determining the optical constant of the substrate, the modeling method of increased layers from single to 3-layer was provided. The results indicate that the Cauchy dispersion model is available for optical transparent or weak-absorption thin films. The structure of the unknown film was fitted with a model of G (1.52)/2.031 2 (203.0 nm) 1.463 6 (170.1 nm) 2.079 1 (170.4 nm) /A by ellipsometry analysis method, and the result was confirmed by the transmission spectrum from a spectrophotometer. The analysis to optical constant and film thickness also show that the ellipsometry is effective for multi-layer thin films analysis.