The flue gas desulfurization(FGD) gypsum-based cementitious materials are prepared by using the β type desulfurization gypsum as the base material, including mixing cement, fly ash, silica fume and self-made modified additive. The effects of raw material composition, water to cementitious material ratio and curing methods on the properties of the FGD gypsum-based cementitious materials have been tested and an-alyzed, and the formation of strength and water resistance mechanism are explored by means of SEM, XRD analysis. The experimental results show that the performance of the FGD gypsum-based cementitious mat-erials is excellent when 20% cement, 25% fly ash are mixed with a water to cementitious material ratio of 0.7. On these conditions,the compressive strength, water absorption and softening coefficient of specimens at the 28th day are 12.1 MPa, 17.6%, and 0.81, respectively. There are two types of skeletons in the specimens made with FGD gypsum-based cementitious materials, which synergistically supports the struc-ture. The first skeleton is formed by dihydrate gypsum crystals, providing the early-age strength for the specimens. The second skeleton is formed by ettringite and C―S―H gel, providing strength in later ages. The skeletons improve the compactness and the water resistance of the specimens made with FGD gypsum-based cementitious materials.