The ELACs of all kinds of components in the FTO (French Test Object) are investigated. And the combined method ( exposure transmission function , peeling onion,and Monte Carlo method ) is used. Many excited results have been worked out. The spectrum is translating and narrowing to the value which is according to the minimal mass attenuation coefficient of the element in the radiography. Because of which, the ELAC decreases while the X-ray goes deeper in the mass. The average efficient linear attenuation coefficient of copper is 0.299(1±3.0%)cm~-1 , while tungsten 0.829(1±2.7%)cm~-1 .With which the high precision density contribution of the components can be got in the reconstruction.