The disclosed positioning system includes welding set, precision 3D servomechanism composed of 3D servomotor and worktable, micro optical unit, microcomputer, and digital pick-up head. The system possesses functions of automatic image recognition and microposition, and operational functions: determining position of welding spot by mouse click, automatic storing calculation, reaching to the designated position, and completing welding. It is possible to modify manual ultrasonic welding machine to automatic welding machine by using the invention. Comparing with manual welding machine, the system possesses advantages: lowering labors intensity, raised work efficiency to 4-6 times, high positioning precision (+ / -0.005mm), good welding quality, and easy of use etc. Comparing with automatic ultrasonic welding machine, the system uses mouse to replace trackball so as to raise positioning precision, lower cost, and provide intuitive operation.