A new method of image segmentation based on neural network was proposed to extract the cracks image, with which it could overcome, in some extent, the dependence of the background image and reduce the pseudopoints and pseudoregions. The measurement accuracy can be improved compared with that from morphological algorithm. Furthermore, the specific methods and processes to measure the walls crack with image digital processing were presented. And with the case study, it was found that this method can measure the cracks accurately with convenient operation.