Abstract: This paper proposed a novel detection method of seam position for realizing arc length control and seam tracking of GTA welding at the same time. In the method, a developed visual sensor is fixed on a welding torch from the side-front view captures image, which includes the front of tungsten electrode, pool, seam lines and the mirror of tungsten electrode in pool. An algorithm based on the constraint of real tungsten electrode center line is proposed to calculate the arc length error. Another algorithm by the hypothesis that the pool being considered as a flat plain is proposed to calculate the seam tracking error. Therefore 3D position of seam relative to tungsten electrode can be obtained as a control input for 3D seam tracking. Experimental results show that the method is valid and the detecting precision reaches ±0.1 mm which meets requirements of precise welding.