Abstract: In order to clarify the hydration characteristics of the composite cementitious system in high performance concrete and boost the application of high performance concrete in water conservancy construction,the cement-slag-fly ash composite cementitious material is used as the research object,and assume the gradient structure low water-cement ratio to be 0.2,0.25 and 0.3,the mortar strength test,chemical combined water method and scanning electron microscope observation,the effects of low water-cement ratio on the mechanical properties,hydration degree and microscopic morphology of high performance concrete cementing system are investigated by using the cement strength test,chemically bound water method and scanning electron microscope observation. The results of the study verifies that,the increase in water-cement ratio causes a linear decrease in the mechanical properties of the system,but the increase in water-cement ratio can significantly mitigate the mechanical loss when the fly ash-slag compounding amount is 40%～50%.The addition of fly ash slag increases the rate of hydration degree in the later stage of the composite system,and the positive hydration effect of pozzolanic reaction is more significant in the system with high water-gel ratio. When the water-binder ratio is 0.2 and 0.25,the growth rate of chemically bonded water and strength is similar.When the water-binder ratio is 0.3,the growth rate of chemically bonded water in the early stage of hydration is higher than that of the strength,but the growth rate of the strength in the late stage is higher than that of the bonded water. When the water-binder ratio is 0.2,the C-S-H morphology is spherically extruded together. When the water-binder ratio is 0.25,C-S-H is layered and stacked layer by layer.When the water-binder ratio is 0.3,C-S-H shows mesh fiber.