Abstract: The ellipsoid is the most common type of primary mirror for the reflective lens. And it is necessary to accurately and conveniently elicit the primary mirror's optical axis in the process of lens alignment, which used as the reference axis of the whole optical system. The traditional optical axis extraction methods mainly rely on a three-coordinate instrument to perform measurement. Due to its low angle fitting precision, contact measurement, and aperture limitation, the conventional optical axis system can no longer meet the requirements of the current large-aperture optical system. To this end, a novel ellipsoidal main mirror optical axis extraction method was proposed. By designing a plane spherical aberration compensator, the non-phase difference surface detection optical path of the ellipsoidal mirror was constructed, and the optical axis of the ellipsoidal mirror was converted into the normal line of compensator, and then the theodolite was used to measure the angular relationship between the normal line and the structural reference, and the main mirror optical axis was elicited indirectly.Results show the optical axis extraction accuracy can reach 1.4″.Compared with traditional optical axis extraction methods, the method has the advantages of higher angular measurement accuracy, non-contact, no size and wider application range, etc.