Abstract: In this paper, the theoretical model describing the static and dynamic relation between the arc current and contact tip to workpiece distance was set up on the basis of gas shielded metal are welding theory. The method to determine the parameters in the model with experiments was studied. It results in a new way of detecting and controlling the vertical position of the torch with the mean current and the transversal position against the welding seam by the swing current of a high speed rotating scanning arc. In the case of a real arc current with great disturbance, the filter and the optimized judgement of position error signal of torch to the welding bead were studied in detail, which aimed at improving the sensitivity, stability and reliablity of the arc sensor.