Research has broken through the technical barriers of active regulation of building solid waste cementitious materials, developed a ternary synergistic activation system of slag fly ash construction waste powder, and revealed the synergistic activation mechanism of multiple solid waste components through multi-scale coupling analysis methods. The results showed that when the ratio of construction waste powder: slag: fly ash mass ratio was 5:3:2, the material performance achieved a breakthrough improvement, with a compressive strength of 75.2 MPa at 28 d, an optimized initial setting time of 60 min, and an expanded flowability of the binder sand to 159 mm, solving the technical problem of synergistic improvement of workability and strength. Microscopic characterization confirms that slag continuously releases active Ca²⁺, while fly ash provides a highly reactive silicon aluminum phase. The dual composite stimulates the depolymerization and recombination of inert silicon oxygen networks in construction waste powder, generating calcium silicate and aluminum calcium silicate crystals, significantly reducing the number of microscopic cracks, improving integrity, uniformity, and compactness. This provides a full chain solution for composition design, interface control, and structural optimization in the construction of high-performance solid waste based cementitious materials, promoting the resource utilization of construction solid waste into a new stage of high value-added applications.