To explore the optimal ratio of limestone powder and calcined clay in the limestone calcined clay composite cementitious material system, the response surface method was adopted to optimize. The prediction model was established by taking ordinary Portland cement, calcined clay and limestone powder as variable factors, and 28 d compressive strength and 28 d flexural strength of cement mortar as response variable. Mechanical properties were tested, and the regression model between the proportion of mineral admixture compounding and the mechanical properties was established. At the conditions of mineral admixture proportion of 15%, 30% and 45% (mass fraction, the same below), the regression model was used to optimize the mixing ratio of limestone powder and calcined clay in the limestone calcined clay composite cementitious material system. The optimization results are as follows: calcined clay accounts for 15%, without limestone powder. Calcined clay accounts for 25%, and limestone powder accounts for 5%. Calcined clay accounts for 30.5%, and limestone powder accounts for 14.5%. The predicted values of 28 d compressive strength of system are 63.26, 58.57 and 48.10 MPa, respectively, while the measured values are 63.74, 57.21 and 47.02 MPa, respectively. The absolute relative errors between the predicted and measured values are less than 5%. It provides a new solution idea and test basis for the optimal proportioning problem of limestone calcined clay composite cementitious material system.