Abstract: As the main raw material of alkali-activated material, the blast furnace slag can produce cement substitute with good cementing property under the condition of suitable activator.In this paper, taking blast furnace slag as main raw material, calcium oxide as alkali-activator and gypsum as additive, discussed the influence of different amount of gypsum dosage on mechanical properties of blast furnace slag activated by Cao and analyzed the reason for improving alkali-activated BF slag cementitious material’s properties by using X ray diffraction(XRD), scanning electron microscopy(SEM) and mercury intrusion method(MIP).The experimental results show that when the gypsum content is 10%, the compressive strength of the cementitious material test block is the best, and the curing 28 days’ can reach 22.20 MPa.The amount of ettringite in the product has a great influence on the strength of the test block, the excess gypsum can’t react with the substance in the reaction system when the amount of gypsum is too high and it is disorderly distributed in the system to form different sizes of pores, eventually leads to the drop of the test block strength.