Abstract: As it cannot integrally divide the defects under the complex background by the traditional measurement methods of the warhead scratch defect size, the method of adaptive threshold segmentation is used to solve this problem. Using this method for image segmentation, each pixel of the image corresponds to the different threshold, it can avoid the error division between defects and background, and the noise interference can also be avoid when the defects are completely divided. Defect segmentation process are given, the defect size is calculated, and the precision of the size measurement is analyzed. Experimental results shows that the defects can be completely divided by this methods when the gray level are different between the defects and the background, standard error is 0.122 8, and the uncertainty is 0.368 4.