Abstract: This paper introduces a low spead rotating mechanism that enables the welding torch to move elliptically and obviously improves the formation of oblique corner joint surface.With this device,the surface shape of welding pool and the dynamic variation of length of arc and wire prolongation were studied.Subsequently,the theoretic relation of the offset of rotating arc center from the groove center to the variation of the welding current was derived.Based on this relation,a new arc-sensing theory was set up by extracting signal from frequency domain with the use of bandpass filtor and correlated lowing noise technique.By this theory,the analog circuits of the welding arc sensing and tracking system were built.The testing results showed that the cross tracking error was only ±0.5mm for CO₂ welding.