Abstract: Dual-energy CT can be used for computing atomic number and electron density. As a result, we can discriminate unknown materials. Spectrum estimation is the fn'st step in dual-energy CT. Its accuracy directly affects dual-energy CT results. However, no systematic studies concerning how spectrum estimation accuracycontributing to reconstruction results have been worked out up to now. This work employs photoelectric-Compton decomposition dual-energy CT reconstruction method. We quantitatively analyze the propagation process from spectrum estimation error to dual-energy CT results and give theoretical calculation method. We propose one way to define spectrum error for dual-energy CT and give the concept of equivalent spectrum. Simulations are shown to support our work.