Abstract: When measuring multi-surface parallel samples, the multi-surface interference can affect the measurement results. To improve the multi-surface interference problem, an equal optical path interferometer was introduced and the optimal method to compensate for the wedge angle of reference mirror was studied. In an equal optical path interferometer, due to the existence of wedge angle and inclination angle of the reference mirror and beam splitter mirror, the retrace errors would occur in the measurement results. The different compensations were applied depending on the different cases of reference mirror wedge angle. The placement and compensation amount of the various compensations were theoretically analyzed and determined, and the effect of the size of inclination angle and wedge angle on the height of the beam to be blocked and the image square NA was studied. The interferometer simulations were then carried out using Zemax with different compensation methods, and the magnitude of the retrace error was analyzed according to the PV values of the interferograms obtained from the simulations. The analysis shows that the optimal compensation method is chosen, with a wedge angle of reference mirror of 1′ and the retrace error is better than 0.003 wavelengths.