Abstract: An image measuring system was designed to measure tip clearance when assembling aero engine. Its camera position was precisely controlled through PID controller, which was based on open control system of PC and motion control card. The camera was calibrated through its accurate movement. The blade tip clearance was measured on line. Compared with other image measuring system and traditional feeler gauge, this system has some advantages: first，it is efficient. Its calibration and measuring process is simple and fast; Second, it can achieve the on-line measurement during the assemble process; Third, when the measuring range is within 1 mm, its error is below 20 m.