In order to explore the mechanical properties of lead smelting slag and metakaolin based composite cementitious materials under sodium silicate alkali excitation, the effects of ball milling time of lead smelting slag, alkali equivalent, alkali activator modulus and the mass ratio of metakaolin to lead smelting slag on the mechanical properties of composite cementitious materials were studied by single factor test and orthogonal test. The hydration mechanism of composite cementitious materials was comprehensively analyzed by XRD, SEM and FTIR. The results show that the order of influence on the 28 d compressive strength of composite cementitious materials is as follows: alkali activator modulus, ball milling time of lead smelting slag, alkali equivalent, mass ratio of metakaolin to lead smelting slag. When the ball milling time of lead smelting slag is 4 h, alkali equivalent is 6% (mass fraction), alkali activator modulus is 1.4, and the mass ratio of metakaolin to lead smelting slag is 3∶7, the 28 d compressive strength of composite cementitious materials reaches 56.18 MPa. Metakaolin promotes the hydration of lead smelting slag and generates more gel and silicaluminate crystals with reticular structure to fill matrix pores, which promotes the late strength development of cementitious system.