Dual-energy X-ray computed tomography (DECT) can accurately reconstruct the effective atomic number and electron density distribution. Currently, it is an effective material identification technology for security inspections. X-ray energy spectrum estimation plays an important role in dual-energy CT. First, the reconstruction principle of dual-energy CT is introduced. Second, based on common transmission data estimation method, this paper focuses on spectrum estimation using CT image reprojection technology. By scanning the cylindrical model which contain two materials and using the maximum expectation algorithm (EM) to estimate the X-ray energy spectrum distribution, the simulated spectrum of Monte Carlo was used as the initial value of iteration. This method improves the accuracy of dual-energy CT reconstruction algorithm.