Phase diversity wavefront sensing technology has been implemented successfully in many domains because of its simple and compact optical configuration, lower environment requirement, better measurement accuracy, and so on. In this paper, the co-phasing technique for segment primary mirrors is studied by using the phase diversity wavefront sensing technology, and the corresponding experimental optical system is also set up. The experimental results show that the measurement accuracy of phase diversity wavefront sensing technology for piston error is better than /20 and that it can be used to detect the piston errors of segment mirrors.