The invention discloses an edge sensor for a segmented mirror surface based on an interference principle, and a working method thereof. The edge sensor is characterized in that: an optical flat or spherical lens is placed on a sub-mirror splicing seam of a measured segmented mirror surface, the front surface of the optical flat or spherical lens is completely anti-reflection, and the rear surfaceof the optical flat or spherical lens is provided with a coating film; a parallel light source is arranged on the other side of a sub-mirror of the measured segmented mirror surface and the optical flat or spherical lens, light rays vertically enter the optical flat or spherical lens by means of a semi-reflecting and semi-transmitting prism and then part of the light beam on the surface returns, the other part is reflected by the surface of the sub-mirror along an original path, the two beams of reflected light form interference fringes, the interference fringes enter a microscopic amplification imaging system by means of the semi-reflecting and semi-transmitting prism, and target surface receiving and digital imaging are carried out by means of a CCD or CMOS detector, namely, the splicingerror of the adjacent sub-mirrors can be handled. The edge sensor prevents starlight and wavefront sensors from occupying and wasting a limited high-imaging-quality field of view of an optical system, is low in price, stable in performance and not affected by environmental factors, and is suitable for detecting the splicing errors between the adjacent sub-mirrors of various optical segmented mirror surfaces.