Starting with the nature of welding seams, the artical studiws Changes of the microstructures and chemical components of welding seams duning the process of welding, then discuss the effects of these changes on active infrared images of reflection. Through theoretical and experimental analyses, it is well known that active infrared technology can be applied in the investigation of industrial welding seams. The method put forth in the article mainly aims at the practical application of industries, and is characterized by rapid, of high precision, on-line in real time. The precision is up to 0.1mm. It takes only 0.02s to segment an active infrared image on a P-700 personal computer.