Accurate determination of the angle of incidence (AOI) of ellipsometer could improve the reliability of the model analysis, thus relieving possible parameter correlation, eliminating inconsistency which is often encountered by using reference samples of different sources to fit the AOI. In this work, firstly, various commercially available standard reference samples are used to retrieve the AOI and compared with each other, which demonstrates the difficulty to make a best choice. Then, the autocollimator-based method is proposed for the establishment of a metrological ellipsometer and the traceable calibration of the AOI is realized. For practical use, a simplified method which is often empirically employed in aligning optical elements is inspected and the validity and accuracy are verified experimentally. The experimental statistics indicate that, the accuracy (deviation from the “true” value) of the mean AOI could be determined in a worst case to be 0.03°, and by optimizing certain geometric conditions, the accuracy could be improved up to 0.0013° with a precision of 0.005° (standard deviation).