X-ray radiographic imaging technology with variable energies acquires useful information of the object under tests in different thickness ranges by setting changing voltage parameters of the tubes. Such information are fused to show the complete internal structure of the object. In practical, the method to determine weighted coefficient of sub graphs based on tube voltage-thickness-device response curves in existing X-ray radiographic imaging experiments is passing out of current use, especially for composite materials. The reason is that the attenuation coefficient of the material and the fluorescence conversion efficiency of phosphor transform screen of materials are related to each other in energy spectra. A method of controlling varying steps of the tube voltages based on relations between adjacent tube voltages and effective transillumination thickness is presented by studying the gray changing rule of transillumination images under different voltages. And then, the proportion histogram is proposed to calculate the weighted coefficient. X-ray imaging of composite objects with variable energies shows experimentally that the mentioned method can realize fusion of effective sub-graphs of composite objects. It overcomes shortcomings of the existing image processing algorithms in X-ray radiographic imaging technology with variable energies.