The influence of slag dosage on the hydration kinetics of slag-cement composite cementing material system is studied.The results show that the exothermic rate of hydration decreases as the slag dosage increases or the temperature of hydration decreases.The non-evaporable water content increases at first and then decreases with the increasing of slag dosage, and reaches the maximum as slag dosage is 30%.The compressive strength decreases as slag dosage increases, the increasing rate of which is evidently before 28 days, after that it is flat or become slow.It can be seen from SEM (scanning electron microscope), in the system of slag dosage below 30%, the gel-like structures bind with crystals which make the system very compact.