Municipal solid waste incineration fly ash (MSWIFA) can be reused as an admixture in cementitious materials, but its low activity limits its utilization as a resource. In this study, we systematically investigated the mineral and grinding characteristics of MSWIFA and then studied its pretreatment and activation via mechanical force–surface modification. The results indicate that the fineness and angle of repose of MSWIFA during grinding are inversely proportional to grinding time, while specific surface area and powder fluidity increase. Agglomeration occurs in the later stage, and particle size fluctuates. Gray correlation analysis shows that MSWIFA powder with a particle size of 16–45 μm contributes most to compressive strength improvement. The composite surface modifier TEA-STPP benefits grinding, shortens ball-milling time, and increases active particle size content, thereby promoting hydration activity. The best process regarding the modifier was determined. MSWIFA and blast furnace slag (BFS) accelerate early hydration of ordinary Portland cement (OPC) and increase its reaction participation, promoting the generation of calcium chloroaluminate (Friedel’s salt) and monosulfate-aluminate phases (SO₄-AFm) and significantly enhancing the hydration of tricalcium aluminate (C₃A) in OPC.