Lower early hydration activity is one of the important factors limiting the application of higher content smelting slag in the cementitious materials system. In this study, silicomanganese slag (SM) was activated by sulfate in circulating fluidized bed combustion ash (CFBA), and the hydration process and activity development of SM-CFBA composite supplementary cementitious materials were studied. Results show that the fluidity of mortars prepared by using SM-CFBA composite supplementary cementitious materials decreases significantly with the increase of CFBA. However, early and later activity of SM-CFBA composite supplementary cementitious materials are effectively improved with the increase of CFBA. When the content of CFBA is 10% (mass fraction) in SM-CFBA composite supplementary cementitious materials, 3 d, 7 d, and 28 d strength activity indexes of them reach 61%, 71%, and 95%, respectively, which is higher than those of SM (3 d, 7 d, and 28 d strength activity indexes are 50%, 53%, and 81%, respectively). The incorporation of CFBA stimulates the early hydration of cement and supplementary cementitious materials, and delays the transformation of ettringite into monosulfur-type hydrated calcium sulfoaluminate (AFm) during the hydration process, which allows early hydration of the cementitious materials to form more ettringite.