The incident low-energy X-ray photon which interacts with CCD detector can produce photoelectric effect and generate photoelectrons. Under the ideal condition, the number of electrons is proportional to the energy of incident X-ray photon. Besides the electrons generated by X-ray absorbed in CCD is converted to readout voltage linearly . Therefore, the voltage is positively correlated with X-ray photon’s energy , and we can measure X-ray energy spectra directly. But in reality, because of the charge cloud diffusion, surface charge loss and charge transfer loss in the detection process based on CCD ， the spectral resolution （FWMH）is broadened. And in the mean time, the spectral peak becomes non-Gaussian with a shoulder and a flat shelf on the low energy side. In this paper , we develop the model and simulation of CCD’s spectral redistribution , including photoelectric conversion process, charge collection process, charge transfer process, and charge readout process. Among other things , we mainly analyze the influence of charge transfer efficiency on X-ray energy spectra.