According to the characteristics that images taken in industrial fields have low resolutions but the detected targets are large on images, a new method for detecting largescale weak edge is presented. It is based on the principle that when the image resolution is reduced, the high frequency characteristics of image are distorted while the lows are highlighted. The weak edges are searched by extracting some subimages, detecting edge on each subimage and integrating the edges of subimages with that of original image. Experimental results show that the method can detect large scale weak edges effectively.