In consideration of the requirements of quality inspection and reverse design for Computed Tomography(CT) products,a CT image edge degradation model was established and used in the image size measurement to improve the measurement accuracy. At first,the model was established based on mathematical description of CT image edge degradation. Then,the parameters of degradation model were confirmed according to the image intensity and inflexions of CT image gray curves. Finally,this constructed degradation model was used to restore the primary CT image edge and accurately extract the information of image edge. Hence,the measurement of CT image is accomplished. Experimental results indicate that the relative error is 2% for the CT image with a poor step edge,which has decreased by 1.5% as compared with those of traditional image measurement methods. These results show that this new model can effectively extract the information of CT image edge,and can get an accurate measurement result.