In order to prepare a composite cementitious material with large amount of phosphogypsum,on the basis of deter-mining the type of main hydration product,by calculating the boundary conditions of the critical ettringite expansion failure,the optimal dosage range of each component was determined and the effect of it on the mechanical properties and dry shrink-age properties of the composite cementitious materials was studied.The formation and development rule of hydration products by XRD and SEM were researched.The results showed that,by theoretical compounding,when the mineral powder dosage was 50%（mass fraction）,the maximum phosphorus gypsum dosage was 26.3%（mass fraction） and the minimum clinker dosage was 23.7%.The optimum ratio group was as follows：28 d,mortar compressive strength was 45.2 MPa,the limit expansion ratio was less than 0.04%.The control group was as follows：28 d,compressive strength was 36.4 MPa,the limit expansion ratio was much greater than 0.04%.The characteristic results of XRD and SEM showed that the hydration products of phosphogypsum-based composite cementitious materials were mainly AFt and C-S-H gels,and Ca（OH）₂ was almost completely reacted.The amount of ettringite produced in the control group was much larger than that of the optimal ratio group,and there were many cracks in the microstructure.The crack indicated that the theoretical ratio calculation can be effectively used to optimize the compatibility of the phosphogypsum-based composite cementitious material.