An imaging system used for material discrimination of an objet is provided. The imaging system includes an X-ray source and an imaging sensor. The X-ray source generates multienergy polychromatic X-ray, wherein there is an integral-multiple relationship between the multi-energy-band of the X-ray. The object is placed between the X-ray source and the imaging sensor. The X-ray is transmitted toward and imaged by the imaging sensor through the object. An imaging multi-energy-band of the imaging sensor corresponds to the multi-energy-band of the X-ray.