The segmented telescope can realize the diffraction-limited imaging when segmented-mirrors are in co-phase. Therefore, an active optical experiment system was built to measure and adjust the phase difference between the two segmented mirrors. The segmented telescope is consists of three hexagonal segments with the side lengths of 300 mm and the radius of curvature of 2 000 mm. Firstly, a Shack-Hartmann sensor and high accuracy micro-positioning stages were used to make the segments co-focused in a high accuracy. Then, a spherometer was used to piston the phase difference between the two segments to a few micrometer level and the principle of white light Fizeau interferometry was utilized to adjust the phase difference. Finally, the sub-aperture diffraction method was taken to measure the phase difference and to piston the mirror to reduce the phase difference to near zero. To verify the calibration effects, an imaging experiment was carried out for an optical bundle, the experimental results show that the measurement accuracy of white light Fizeau interferometry is better than 100 nm, and that of sub-aperture diffraction method is better than 16 nm, which demonstrates that the methods introduced are suitable for the co-phase calibration of segmented telescopes.