Segmented mirror co-phasing is one of the key techniques in the development of large-aperture telescopes, and its accuracy mainly depends on the measurement accuracy of the edge sensors. Aiming at the co-phasing requirement of large-aperture segmented mirror telescopes, we proposed the design scheme of an edge sensor based on equal thickness interference. Then, the basic principle was expounded and the structure diagram of the edge sensor was provided. Furthermore, data fitting and image processing were applied to the scheme simulation. The results show that the measurement accuracy of this scheme can reach a tilt/tip error of 0.02″ and a piston error of 20 nm, meeting the co-phasing detection requirements of large-aperture segmented mirror telescopes.