During interferometric testing, an accurate evaluation of the surface information is one of the key factors in realizing deterministic manufacturing of off-axis aspheric mirrors. The method used in the analysis of wavefront aberration due to centring error in optical system is valid for test piece misalignment in an interferometric setting as well. The mathematic model of error separation is developed. In experiment, the test result of an off-axis aspheric mirror after alignment is 0.037λ (RMS, λ=0.6328 μm), the test efficiency is improved effectively.