A method and system for dual-energy X-ray CT are disclosed. In the method, two effects which are predominant, such as the Compton effect and the pair effect, are maintained, and influences of other effects such as the photoelectric effect are eliminated, thus increasing the material decomposition precision. The method and the system are advantageous in that errors caused by direct selection of two effect equations for atomic number Z calculation in material decomposition processes in present dual-energy CT (regardless of keV low energy or MeV high energy) methods can be effectively eliminated, thus greatly improving material decomposition and recognition accuracy of dual-energy CT; and the method and the system are of great significance for applications in clinical medicine, safety inspection, industrial nondestructive testing, and other fields.