1. A method for substance identification comprising steps of: transmitting an object under inspection using high-energy rays and low-energy rays to obtain a high-energy transmission image and a low-energy transmission image for the object, wherein a value of each pixel in the high-energy image indicates a high-energy transparency of the high-energy rays with respect to corresponding parts of the object, and a value of each pixel in the low-energy image indicates a low-energy transparency of the low-energy rays with respect to corresponding parts of the object; calculating a value of a first function for the high-energy transparency and a value of a second function for the high-energy transparency and the low-energy transparency, for each pixel; and classifying locations determined by the value of the first function and the value of the second function using a pre-created classification curve, so as to identify the type of the substance of a part of the object corresponding to each pixel, wherein the classification curve is created for each type of calibration material through the following steps: obtaining corresponding high-energy transparency and low-energy transparency by irradiating calibration materials with various thicknesses using the high-energy rays and the low-energy rays; forming points of the calibration materials with difference thicknesses by taking the first function of the high-energy transparency as an abscissa and the second function of the low-energy transparency and the high-energy transparency as an ordinate; and forming the classification curve based on the points.