In this paper, we propose a method which make it possible to investigate the films with complex refraction index using spectroscopic ellipsometry. By introducing a new aiming function, the search parameters space can be reduced to 4-dimension, so we not only can calculate the optical constant and the thickness of the films, but also the optical constant of the substrate simultaneously. Using this method, we studied the optieal constant dispersion and the growth rate of ITO films which is deposited on Si substrate by sputtering. At the same time, we observed that the apparent optical constants of the Si substrate were changed.