In cone-beam 3D CT, 2D X-ray projections determine the reconstruction quality directly. However, the generation of an X-ray projection is a complex process that includes the emission, the conversion and the transfer of X-ray latent information, so the pixel value of a projection has no exact meaning generally. In this paper, we suggest a way by experiments to model the whole imaging system based on 'equivalent single energy' which is used to analyze polychromatic X-ray. We define each tissue's SCF(single calibration factor) of human and get GCF (gross calibration factor) by summing up every weighted SCF, then we can use GCF to calibrate and linearize X-ray projections. After such processing, X-ray projection will have explicit meaning, i.e. its pixel value can denote the equivalent material thickness linearly, which will be advantageous to the algorithm research of 3D reconstruction and other quantitative analyses of human's physiological parameters.