Abstract: It is very difficult to detect molten pool width by directly acquiring its in underwater welding.Based on strip-structured weld image, a on-line detection method of molten pool width is proposed.The laser-structured vision sensor is fixed behind the welding torch and put into water with waterproof guard.According to the feature of underwater weld image, the molten pool width is extracted through window processing, laplace sharpening, thresholding, mathematic morphology filtering and thinning procedure.The experimental results show that the processing speed is fast and the detection precision is high.If the distance between the molten pool centre and the strip light detecting point is less than 35 mm, the comparative error of molten pool width can be controlled within 3%.