Abstract: Theoretical analysis and digital simulation of weld tracking system with a leading sensor are stated in this paper.Based on the analyzing of movement process of the tracking system,a loop control structure diagram of the system and its simplified digital dispersed control diagram are presented,which makes the microcomputer control program of the tracking system much simple.The results of simulation tests show that the above digital controller has good behavior and can respond to a variety of input signal exactly,such as a jump,a slope and a folded line signal,in 2~3 steps.