The primary mirror's aperture limits the telescope's resolution. A segmented primary mirror is adopted to improve the
resolution. Since the primary mirror with large diameter is segmented, the segmented mirrors must be co-phased to
ensure the imaging quality. Optical method and electrics method can be used to test the co-phasing error between the two
adjacent segment mirrors. However the optical system of telescope will be complicated by attaching an optical cophasing
test system, and some unstable factors will be introduced. In this paper, an electrics co-phasing testing method
was discussed. Capacitance micrometer technology was used in this method, and relevant experiments have been carried
out. And the result of these experiments indicated that the method described in this paper is feasible.