Abstract: Based on a brief introduction of the basic principle of ellipsometry and some derivations, the sensitivity of ellipsometric parameters with respect to film parameters and the influence of incident angle on the ellipsometric parameters were discussed, and its simulation analysis was made. It shows that the sensitivity of ellipsometric parameter Delta to the film optical constants and film thickness is apparently higher than another ellipsometric parameter Psi, and the measurement accuracy of ellipsometric parameter Delta directly affects the fitting accuracy of film optical constants and film thickness in the ellipsometric data processing. A method is provided to improve the measurement accuracy of Delta by choosing the incident angle close to the Pseudo-Brewster Angle.