CT imaging system comprising: a X-ray source ; a first detection and collection unit which is disposed to be opposite to the X-ray source and configured to perform a first CT scanning on an object to be detected to obtain scanning data; a reconstruction unit configured to reconstruct an image of a first resolution based on the scanning data; a second detection and collection unit which includes at least a photon counter detector and is configured to perform a second CT scanning on an area of interest in the image of the first resolution, the second CT scanning including scanning on the area of interest under a plurality of energy windows by the photon counter detector to obtain truncated energy spectrum data; wherein the reconstruction unit is further configured to supplement the truncated energy spectrum data by using the image of the first resolution as a priori image, and to reconstruct an image of a second resolution from the supplemented energy spectrum data, the second resolution being higher than the first resolution; the CT imaging system being characterized in that the second detection and collection unit is further configured to reconstruct an image of attenuation coefficients under the plurality of energy windows at the second resolution, a basis-function decomposition post-processing is performed on the image of attenuation coefficients under the plurality of energy windows to obtain decomposition coefficients under different effects, and atomic numbers and an electron density map of the area of interest are calculated thereby, wherein a number of basis functions, no more than the number of energy windows, are chosen in performing the basis function decomposition. a X-ray source ; a first detection and collection unit which is disposed to be opposite to the X-ray source and configured to perform a first CT scanning on an object to be detected to obtain scanning data; a reconstruction unit configured to reconstruct an image of a first resolution based on the scanning data; a second detection and collection unit which includes at least a photon counter detector and is configured to perform a second CT scanning on an area of interest in the image of the first resolution, the second CT scanning including scanning on the area of interest under a plurality of energy windows by the photon counter detector to obtain truncated energy spectrum data; wherein the reconstruction unit is further configured to supplement the truncated energy spectrum data by using the image of the first resolution as a priori image, and to reconstruct an image of a second resolution from the supplemented energy spectrum data, the second resolution being higher than the first resolution; the CT imaging system being characterized in that the second detection and collection unit is further configured to reconstruct an image of attenuation coefficients under the plurality of energy windows at the second resolution, a basis-function decomposition post-processing is performed on the image of attenuation coefficients under the plurality of energy windows to obtain decomposition coefficients under different effects, and atomic numbers and an electron density map of the area of interest are calculated thereby, wherein a number of basis functions, no more than the number of energy windows, are chosen in performing the basis function decomposition. <EOS>