In the conventional X-ray imaging, the ray image of the complicated structural component (shape, structure complex and multi-materials) easily exhibits the overexposed and underexposed phenomenon. This is because of the bigger variations in the effective thickness in the orientation of X-ray penetration and the limit of the dynamic range of X-ray imaging system. The complete structure information can’t be obtained, and it will impact the quality of X-ray CT image. So the present paper has improved the theory and method of variable voltage X-ray multi-spectrum CT imaging. The new method changes the distribution of X-ray spectrum by changing the voltage, and can realize the matching between the X-ray spectrum and the effective thickness of workpieces. This can expand the dynamic range and ensure the integrity of projection about the workpieces. At the same time, based on the idea of virtual design, the paper has built the variable voltage virtual CT mode based on single-energy and multi-energy, and realized virtual simulation of the variable voltage X-ray CT. At last, the virtual experiment has proved that the theory and method of variable voltage CT imaging is feasible, and the variable voltage CT imaging is the effective method for the complicated structural component.