Abstract: A photo electic sensor with a scanning spot light source was used to detect the seam error. Based on features of the sensor' s signal, a comparing method of weighted signal was presented for seam tracking.A methematical model of rotating sensor was set up, and was analyzed theoretically.A fuzzy cotroller was designed, and was simulated with MATLAB.A seam tracking system based on MSC-51 single-chip microcomputer was designed. The results of welding test show that the system is accurate, stable and low cost, and can meet the requirments of prodution.