To realize a time-efficient and wider range of measurements of piston errors of segmented mirror, a method based on the coherence of the Fraunhofer diffraction pattern of two half circles in visible light was proposed to detect the piston errors of the segmented mirror. In this method, the noncoherent diffraction pattern of the two half circulars was used as a template, and the cross-correlation algorithm was used to calculate the cross-correlation coefficient between the template pattern and the actual diffraction pattern. To achieve coarse co-phasing detection of piston errors of the segmented mirror, the threshold value of the cross correlation coefficient was set as 0.85. The proposed method has advantages such as time-efficiency, better energy usage ratio, infinite detection range, and a better measurement accuracy. The method was validated by theoretical, analytical, and numerical simulations. An active optics and co-phasing experimental system of the segmented mirror was built to measure and adjust the piston errors of the segmented mirror. The segmented mirror consists of four hexagonal segments whose edge to edge length is 100 mm and the curvature radius is 2 000 mm. Initially, to fine co-focus the segmented mirror, a shack-Hartmann sensor and active optics technology were used. Then, the coherence of the Fraunhofer diffraction pattern of the two half circulars in visible light was used to detect the piston errors of the segmented mirror, which can be adjusted using active optics technology. The experimental results show that the detection range is infinite and the measurement accuracy is better than ±250 nm. Theoretical, analytical, and experimental results demonstrated that the proposed method is suitable for segmented mirror coarse co-phasing measurement and adjustment.