Abstract: It is necessary for the improvement of weld quality and the realization of automatic production to detect weld formation parameters automatically.Automatic detective system of weld formation parameters by back CCD visual sensor is developed.The grey histogram of back seam image shows three peaks.The procedure of image process is as follows:twice automatic extractions of region threshold by iteration method, three thresholds processing, and inner noise elimination and so on.The system can detect the dimension parameters of molten and heat effect zone accurately.The experiment shows that this method is to improve the stability of weld production and to meet the demands of industry without the interferences of arc, spatter and electromagnetism, etc.