Abstract: Backfilling and grouting in the goaf are effective methods that can efficiently dispose of solid wastes including coal gangue (CG) and coal gasification slag (CGS). When backfilling is solely for solid waste disposal, the strength requirement for backfill materials is low. In view of this, a gangue and coal gasification slag-based backfill material (GCBM) was prepared, using a low content of alkali-activated slag (AAS) to adjust its mechanical properties. Considering three influencing factors (solid content, CGS content, and AAS content), single-factor experiments and optimization experiments based on response surface methodology (RSM) were conducted, with fluidity and strength as the optimization objectives. Finally, the hardening mechanism and microstructure of GCBM were analyzed. Test results show that the fluidity of GCBM is negatively correlated with solid content, CGS content, and AAS content; the strength is positively correlated with solid content (in a certain range) and AAS content, and first increases and then decreases with the increment of CGS content. The optimal mix-proportions obtained via RSM were as follows: 75.35wt% solid content, 24.13wt% CGS content, and 4.94wt% AAS content. Analysis of GCBM’s chemical composition and microstructure indicates that its main gel phases are calcium aluminosilicate hydrate (C–A–S–H), sodium aluminosilicate hydrate (N–A–S–H), and potassium aluminosilicate hydrate (K–A–S–H). The gels begin to adhere to and wrap inert solid particles when elements Si, Al, Ca, Na, and K dissolved in the alkali-activated system undergo heterogeneous nucleation on solid particle surfaces. The presence of multiple spherical pits on the fracture planes suggests that glass microspheres in CGS to some extent reduce GCBM’s strength. The specific surface area and pore structure of GCBM were analyzed, and its environmental safety was preliminarily verified. The results provide theoretical guidance for the large-scale, efficient backfilling disposal of solid wastes, especially CG and CGS.