For segmented primary mirror of extremely large telescope, in order to make the image quality of telescope system reach the diffraction limit, sub-mirrors positioning accuracy requires to meet the requirements of co-phase. So the piston errors between sub-mirrors need accurate measurements and corrections in real time. On the basis of analyzing dispersed fringe sensing (DFS) and disperse Hartmann sensing (DHS) principles, a dispersed fringe sensor was built and some detection experiments on an indoor segmented mirror active optics experimental system were carried out. A certain relation is confirmed between the sensing result and relative piston error value of sub-mirrors. So piston errors can be detected indirectly. And experimental results were analyzed for error factors, which affect the measurement accuracy greatly. They are fringe visibility, spectral curve calibration, nonlinear least squares and so on. During the real test experiments, solutions are also proposed to improve the data processing. The results indicate that the present measurement accuracy is up to 20 nm in the range of ±15 μm.