Most of the common-used methods for cophasing are summarized and their applicabilities for using in space are discussed. A novel method which is based on dispersed Rayleigh interferometry (DRI) is proposed with data processing algorithm. The basic theory of DRI is introduced and the configuration to measure cophasing error of segmented primary mirror is given. An experimental system to validate the DRI algorithm is established, preliminary results indicate that the measurement range reaches 200 μm, repeatability is better than 2 nm，and accuracy is 6.56 nm with the range of δ≤1 μm, which meet the requirements of cophasing the primary mirror in the orbit.