The invention discloses a method and a device for identifying substances. The method comprises the following steps: an object to be tested is transmitted by a high-energy ray and a low-energy ray, so as to obtain a high-energy transmission image and a low-energy transmission image of the object to be tested; the value of every pixel in the high-energy image represents the high-energy transparency of the high-energy ray to a corresponding part of the object to be tested; the value of every pixel in the low-energy image represents the low-energy transparency of the low-energy ray to the corresponding part of the object to be tested; according to every pixel, the value of a first function of the high-energy transparency and the values of second first functions of the high-energy transparency and the low-energy transparency are calculated; and by utilizing a classification curve created in advance, positions determined by the value of the first function and the values of the second functions are classified, so as to identify the types of the parts in the object to be tested, which are corresponding to every pixel. The method and the device not only can obtain the transmission images of the object to be tested, but also can obtain the material information of the object to be tested.