A method for visual measurement was proposed to acquire welding formation sizes based on laser line scanning. A visual system for detecting welding formation was developed and simultaneously calibrated, based on which, images of welding formation and laser stripe which were cast on the surface of weld were captured in real time. And extracting and thinning of laser stripes in the image were accomplished by means of a series of image preprocessing methods, such as median filtering, threshold segmentation, skeletonization and morphological processing. The laser stripe being made as information source, the outline of weld forming was revealed exactly, based on which, a series of algorithms for realtime detection of the width and length of weld were established. The measured results show that visual measurement method aided by laser stripe can acquire the width and length of weld automatically, and the measured width and length of weld can meet the demand of weld formation evaluation.