Large-aperture collimator was widely used in calibration and testing for various optical devices as a essential equipment for testing and calibration in laboratory. However, when collimator was defocused because of vibration, gradient change of temperature and airflow, divergence of beam which was emitted from collimator was increased and result credibility of test was decreased. To solve the above problems, a new miniature auto-focusing system was presented which used pentaprism for a light feedback element. CCD was used as image receiving device, and software was applied to realize calculation of the system defocusing amount, which can monitor and test the focal plane of collimator on a high precision level for a long time. Computer simulation and test show that the system can reach 45μm auto-focusing resolution and fully meet the precision requirements for laboratory testing equipment.