The X-ray tube of usual computed tomography imaging system produces X-ray beam with a polychromatic spectrum, which results in low contrast of the materials with approximate attenuation coefficients in the reconstructed image. A method was proposed to obtain a new projection by decomposition of multi-voltage X-ray images. The distinction of the materials was enlarged in the reconstructed image by the new projection. In this method, the X-ray image of each voltage was considered as a sum of many narrow-energy-width X-ray images according to the law of X-ray imaging. Referring to non-negative matrix factorization, the new projection was got by minimizing the sum of squared error of multi-voltage X-ray images’ decomposition. A cylinder composed of silicon and aluminum was used in the experiment. The contrast of silicon and aluminum is evidently improved in the new reconstructed image compared to the direct reconstructed image. It shows the effectiveness of the proposed method.