Fly ash, flue gas desulfurization gypsum and cement can be blended into a new composite binder with some activators, and the basic mix proportions of composite binder were determined by selective test. The hydration process of a typical mixture of fly ash flue gas desulfurization gypsum cement paste with composite activators was investigated. It can be concluded that in the hydration process the early age pozzolana activity of fly ash obviously improves, and flue gas desulfurization gypsum not only crystallizes itself to strengthen the paste microstructure, but also acts as an ideal sulfate activator to fly ash. Fly ash begins to hydrate at the age of 3 d, due to activation by the flue gas desulfurization gypsum.