Abstract: As the basic visual morphological feature of molten pool, contour extraction plays an important role in on-line monitoring of welding quality. A molten pool visual sensing system under the environment of tungsten inert gas welding (TIG) welding process is established, and high-quality molten pool images are collected. Aiming at the difficulty of weak edge detection in TIG welding stainless steel molten pool image, a molten pool contour extraction algorithm based on operator template matching based on edge direction guidance (OTM-EDG) is proposed. Firstly, the algorithm enhances the weak edge based on the nonlinear gray transformation method. Then, the Sobel operator in four directions is used to convolute with the molten pool image to judge the direction of the back-end weak edge and calculate the gradient map. Finally, the edge connection operation and edge smoothing operation based on mathematical morphology are carried out on the gradient map to obtain the molten pool contour to be extracted. Experiments show that the algorithm can extract the closed, complete and accurate molten pool contour of TIG welding stainless steel. It has high robustness in the actual welding environment, and effectively solves the problem that the weak edge of the molten pool area is difficult to accurately detect.