Based on the analysis of the relationship between the photon intensity received by the detector under the common energy segment corresponding to different energy spectra, a multi-spectral computed tomography (CT) imaging method using subtraction fusion is proposed herein. From the analysis of the correlation across the X-ray spectrum using different X-ray source parameters, we establish a subtraction fusion model involving various energy polychromatic projection sequences and obtain multiple sequences of the phantom by varying energy. Combined with the subtraction fusion model, the projection is obtained at different energies, the projection information of the common energy segments is removed using subtraction fusion, and the projection information of the approximate narrow spectrum is derived. Finally, the expectation-maximization/total-variation algorithm is used to reconstruct the image, reduce the noise interference, and improve the reconstruction quality. Theoretical analysis and experiments show that the proposed method can obtain approximate narrow-spectrum projection through multi-spectral projection subtraction fusion, which can effectively suppress beam hardening artifacts and improve CT imaging quality.