The invention discloses an image processing based X ray welding line zone extracting method and relates to the field of industrial automatic detection. The method includes a step A of image pretreatment; a step B of edge detection based on least squares line fitting; a step C of scale product calculation. The invention has beneficial effects that noise interference can be suppressed prominently and edge detection accuracy can be improved. Therefore, good accuracy and robustness can be achieved in application in X ray images with a large amount of noises, low comparison degree, blur welding line zone edges and high grey degree distribution overlapping of the welding line zone and the background.