1. A dual-energy material identification method with under-sampling, comprising steps of: CT scanning an object under inspection with ray beams at a first energy level to obtain projection data at respective angles at the first energy level, and reconstructing a CT image of the object; CT scanning the object with ray beams at a second energy level to obtain projection data at a part of the respective angles at the second energy level; combining the projection data at the first and second energy levels to obtain dual-energy under-sampled data at the part of the respective angles; acquiring a photoelectric coefficient integral value and a Compton coefficient integral value from the dual-energy under-sampled data; segmenting the CT image of the object into a plurality of regions and computing a length by which the dual-energy rays cross each of the regions; computing the photoelectric coefficient and the Compton coefficient by way of dual-energy preprocessing dual-effect decomposition reconstruction method, based on the lengths of the rays crossing the regions, the photoelectric coefficient integral value and the Compton coefficient integral value; computing at least atomic number of material in each of the regions based on the photoelectric coefficient and the Compton coefficient; and identifying the material of the object based on at least the atomic number.