The calculation formula of reflector ellipsometers with variable incidence angles and its solution were disussed firstly. One kind ofOverall optimization algorithm was proposed, so that the multi-peak problem in solutions was resolved, and the refractive index and thickness of thin films can be calculated in high precision. Then by this new algorithm, the refractive index and thickness of SiO2 film have been calculated easily from the extinction parameters measured under large incidence angles. The results show more accuracy than that of other solutions, that benefits for studying μm～nm film op ticalwaveguideswith complex structrues and compositions. At last some reasons for the bad results under small incident angles was presented