An green ultrafine and high active mineral admixture was developed through the synergistic utilization of various industrial solid wastes. This study systematically investigated the effects of raw material ratios and milling parameters on the performance of admixture, with hydration products and microstructural characteristics analyzed using XRD, TGA, and SEM techniques. The results demonstrate that when water quenched manganese slag, steel slag, fly ash, and desulfurization gypsum are mixed in a mass ratio of 8∶3∶8∶1 with 0.10% (mass fraction) of a milling aid and jointly ground to a specific surface area of 800 m²/kg, the admixture achieves a 7 d activity index of 97.4% and a 28 d activity index of 110.0%, along with a flowability ratio of 95.1%. In the early stage, the admixture minimizes strength loss through physical filling and the formation of calcium aluminate hydrates, while in the later stage, synergistic hydration and pozzolanic reactions enhance strength development. The research results are of great significance for realizing high value-added utilization of low active solid waste and sustainable development of building materials industry.