This study addresses the challenges of high costs, low residual strength, and underutilization of industrial waste in high-water filling materials. It explores the modification of these materials using high-activity slag powder. Key findings include: (1) Slag powder is alkaline and highly reactive, with the main hydration products being ettringite and calcium silicate hydrate (C-S-H). Increasing the water-cement ratio decreases the relative content of effective components, affecting material properties. (2) Adding an optimal amount of slag powder improves the mechanical strength of the high-water materials. Different doping methods can increase slag content effectively. (3) A quadratic polynomial regression model, based on response surface methodology, was developed to optimize the water-cement ratio and slag powder contents for improved compressive strength. The model's accuracy was validated through multiple tests. (4) XRD, SEM, and thermogravimetric analyses revealed performance changes and the curing mechanism of the modified materials. (5) A crystal model for hydration products was established, and molecular dynamics calculations showed that C-S-H has a higher elastic modulus than ettringite, and its deformation resistance is influenced by the calcium-to-silicon ratio. The research provides insights into optimizing high-water filling materials with slag powder for better performance and cost-effectiveness.