Abstract: In order to get a sound weld in narrow gap welding, a new tracking sensor, the photoelectrical digit sensor, has been developed, This sensor has the advantages of high controlling accuracy, less space taking up, simply constructed circuit and powerful anti-interference. With the help of the edge effect of I-groove, the sensor can convert the continuous photosignal into digit signal and get the deviation of welding torch quantitatively. By combining the new sensor with a microcomputer as the main body, a real-time tracking control system can be made. This system effectively ensures high accuracy tracking and complete sidewall fusion in narrow gap welding.