Abstract: Due to the influence of other factors,it is difficult to analyze damage measurements of materials directly using CT image.In this paper,firstly,beginning from computing angle of mass attenuation coefficient of mixture.By analyzing the calculation principle of mass attenuation coefficient.Getting the other one equivalent expressing;The formula among density variations induced by microcracks.CT image value and volume percent of crack defects is obtained under service environment;finally obtained the formula between volume percent of crack defects and damage variable.This lay a theoretical foundation for material fatigue life prediction by using CT.