Abstract: A new algorithm to extract the weld torch lateral deviation for through-arc sensor with sinusoidal weave motion was proposed in this paper.Based on the detailed analysis of the welding current waveform in through-arc sensor with sinusoidal weave motion, the lateral deviation expression was deducted following the left-right area difference method.Adopting the function approximation theory, an index related to the lateral deviation was defined and the optimum result was calculated under this least mean square index.Experimental result proved the validity of the proposed algorithm.These results established the foundation for applications of the through-arc sensor in seam tracking system.