In view of projectile velocity measurement of air gun, a design of correlative laser velocity measurement system was presented. The optical system that consists of semiconductor lasers and photodiodes was used to identify the information of projectile penetrating the target, photoelectric conversion circuit and hysteretic comparator was designed to amplify and shape the signal, instead of computer and the data acquisition, the STC89C52 MCU was used to process the signal. The actual velocity test result for the projectile with 30 mm diameter and 150 mm length shows that the system has advantages of low cost, easy operation, fast response and small error and has a good practical application value.