Abstract: In order to accurately measure the tilt amount of the secondary mirror in coaxial three reverse camera, a new method for measuring angle was proposed, which used the combination of a large-aperture interferometer and a theodolite. Taking the primary mirror as the measuring reference, while the included angle of two mirrors was small, the large-aperture interferemeter was used to measure the interference fringes of two mirror simultaneously. While the included angle was too large and the secondary mirror did not involve the interference fringes, a theodolite was added for autocollimation from the interferometer and secondary mirror to measure the included angle of two mirrors indirectly. Examples and analog computations show that for the secondary mirror tilt angle, its measuring error can be within 0.5″. The results indicate that the measurement method has the characteristics of strong universality and high precision, and can overcome the problem of insufficient measurement accuracy of traditional detection method.