In order to make the resolution of segmented telescopes close to the diffraction limit of equivalent aperture, each sub-mirror must have extra high coplanar precision. In this paper, aiming at the co-phasing problem of segmented telescopes, first, the principle of co-phasing detection technology of pupil plane and focal plane in segmented telescopes optical co-phasing detection technology is studied. Then, the advantages and disadvantages, application fields and future development trends are summarized. Finally, solutions are provided on the detection problems of large piston error and large tip-tilt error respectively.