In this study, a high content fly ash (m(slag powder)∶m(fly ash)=1∶3 (mass ratio)) was used as matrix, and the composite activators under different compounding methods were obtained by orthogonal test design. The effects of different single-doped and co-doped activators on the mechanical properties and hydration mechanism of high content fly ash cementitious system were studied by XRD, FT-IR, TG-DTG, SEM and other test methods. The results show that Na₂SiO₃ has the best activation effect on the high content fly ash composite cementitious system in the single-doped activator. When Na₂SiO₂ content is 1.0% (mass fraction), 7 and 28 d compressive strength of high content fly ash composite cementitious system is 17.5 and 31.7 MPa, respectively. The optimal mass ratio of multiple activators to the excitation effect of high content fly ash cementitious system is m(slag powder)∶m(fly ash)=1∶3. The compressive strength of 7 and 14 d is 27.5 and 35.5 MPa, respectively. The compressive strength of 28 d is increased to 49.2 MPa, reaching the strength index of 42.5 cement. This is due to the secondary pozzolanic reaction of the active substances of fly ash in alkaline environment, and the hydration products such as AFt and C-(A)-S-H gradually increase, which significantly enhances the compressive strength of the cementitious material. This study not only provides a new way for the high-content utilization of fly ash, but also provides a reference for the research and development of solid waste composite materials.