In the measurement of film thickness by extinction ellipsometry， the angle of incidence， as an important process parameter that changes the polarization states of incident and reflected light waves， significantly affects the accuracy of film thickness measurements. A He-Ne laser with a wavelength of 632.8 nm is used to establish a mathematical model between incident angle deviation and measured film thickness deviation under different incident angles. The model is verified by constructing an extinction ellipsometry setup and testing a silica-on-silicon sample. For the measured sample with a film thickness of 102.55 nm， theoretical calculations show that every 0.01° variation in the incident angle introduces a maximum error of 0.008 7 nm to the measured film thickness. Measurement results demonstrate that the actual error caused by incident angle deviation is consistent with model expectations， which further validates the reliability of the extinction ellipsometry setup. The analysis method for the uncertainty component introduced by incident angle deviation in extinction ellipsometry is optimized. The uncertainty component introduced by the incident angle of the setup is 0.004 nm.