An energy spectrum modulation apparatus and a material discrimination method and device are disclosed which can discriminate the material in large- and medium-sized objects (7) such as cargo containers, air cargo containers, etc. by using X-rays having different energy levels. The energy spectrum modulation comprises a rotating disc carrying two filters which filter the radiation beam alternately. The beam energy is also varied cyclically respectively between two discrete levels by varying the acceleration voltage. The invention is thus able to obtain different images using different radiation spectra to distinguish or identify materials in objects being imaged.