To achieve high-precision detection of surface shapes of off-axis elliptical cylindrical mirrors, this paper proposed hybrid interferometry integrating the aberration-free point method and the computer-generated hologram (CGH) method. In view of the special surface shapes of off-axis elliptical cylindrical mirrors, the emergent cylindrical wave of a typical cylindrical (TC) mirror, a highly integrated element composed of a plane mirror and a CGH, was used as the detection light. The optical axis was oriented to be in alignment with the line connecting the elliptical focus on the incident side and the center of the off-axis elliptical cylindrical mirror for a smaller relative aperture of the measurement optical path. Then, interferometry was implemented through the pair of aberration-free conjugate points of the ellipse. With the off-axis elliptical cylindrical mirror regarded as a spatial rigid body with six degrees of freedom on the optical axis, an error separation matrix was deduced. The wavefront aberration theory was applied to derive the parameters of the alignment error brought by the alignment amount and to determine the alignment amount in interferometry. The experimental results show that this method offers an effective measurement of the surface shape of the off-axis elliptical cylindrical mirror, and the parameters of the alignment error can be deduced via an error separation matrix, which paves the way for further analysis and correction of the system error.