In order to reduce the impact of the detector out-of-focus problem on the laser rangefinder system, an active rangefinder detector adjustment system based on split-image mirror was designed. Firstly, the imaging principle of the split-image mirror was analyzed, and its the detection accuracy was theoretically analyzed, and a mathematical model of split-image defocus was established; Secondly, in view of the error factors that affected the detection, the error curve under different values was given, and the change trend of the detection accuracy was obtained; Finally, the theoretical prototype was designed and verified by experiments. The experimental results show that when the defocus is 0-6 mm, the detection accuracy can reach 0.07 mm, close to the ideal value, and the smaller the defocus, the higher the defocus detection accuracy, which is in line with the installation and measurement habits. The defocus detection system provides a new method for the adjustment of non imaging photoelectric detector, which can further improve the measurement accuracy of laser rangefinder system.