Aimed at the difficult problem of segmented telescope co-phasing, a method of co-phasing error measurement based on dispersed fringe sensing technology is proposed. According to the detecting principle of the dispersed fringe sensing technology, an optical imaging model of dispersed fringe sensor is introduced. The simulation study is carried out on the computer. An auxiliary scheme called dispersed Hartmann measurement is brought out to solve the problem that dispersed fringe sensing algorithm loses efficiency under the near co-phasing condition. Simulations are also made to verify the feasibility of this method. By combining the two methods, dispersed fringe sensor can detect piston error under the range of ±60 μm in the visible waveband, and the detecting accuracy can achieve λ/10. Besides, the impacts of various factors on the detection accuracy have been analyzed in quantity. And solutions such as the fringe windowing, multi-trace and enhancing wavelength calibration accuracy are put forward to improve the dispersed fringe sensing technology performance. An advanced algorithm is also presented to make the method less sensitive to calibration errors. The results show that the proposed method can effectively detect piston error with high precision, no blind area and extremely wide range. It can be widely applied in the coarse co-phasing calibration and phasing control for space and ground based segmented mirror telescopes.