Abstract: The digital controller of automatic seam tracking microcomputer system is studied in this paper. In this kind of system the detecting point is usually leading to the welding point for the most of sensors except the welding arc sensor. The following three types of digital controllers have been built and analyzed for the welding tracking system with a leading sensor:minimum step digital controller, digital PID controller and so-called practical digital controller. The results of simulation tests show that the practical digital controller presented by authors is a reasonable control model for the microcomputer seam tracking system with leading sensor,because of its highest tracing accuracy (the tracing error<0.5 mm) and its simplest structure to simplify the calculating of the control variables (the close loop transfer function of the microcomputer control system equals 1).