A co-focus experiment system of segmented-mirror active optics is built to realize the co-focus fine-tuning of a segmented-mirror.In this system, the segmented-mirror consists of three hexagonal submirrors with 300 mm side to side, and the shape of each submirror is spherical with 2 000 mm radius.A Shack-Hartmann sensor is used for co-focus measurement, and six micro-displacement translation stages in this system are used for adjusting the co-focus and tilt of the two sub mirrors.Each sub-mirror corresponds to 36 sub-apertures of the Shack-Hartmann sensor,and the co-focus errors can be calculated by the position deviation of the spots formed by the sub-apertures.After fine-tuning for micro-displacement translation stages, the rms de-focus error in the optical axis direction and the rms tilt errors in two orthogonal dimentions of the sub mirror can be controlled to less than 1 μm and 0.02″,respectivelly.The experiment shows that the method used in this system is also compatible to the co-focus calibration and real time adjustment of the large segmented mirror telescope.