The separation of misalignment aberrations is a crucial step in interferometric testing for the acquisition of the real surface. In this paper, a Fizeau interference system and a conical mirror with a certain angle were used to achieve the shape measurement of the inner surface of the frustum of a cone. Based on the ray-tracing method, the relationship between the adjustment errors and misalignment aberrations was established, and the misalignment aberrations could be removed by using the error separation model that accords with the least-square algorithm theory. The simulation and practical measurement results indicate that the approach proposed is valid and feasible.