The geopolymer composites of coal gangue-slag were prepared with sodium hydroxide (NH) and sodium silicate (NS) as activator. The effect of slag content and NH modules on the fluidity of cement pastes and compressive strength of alkali-activated coal gangue-slag (AACGS) cementitious materials was investigated. The property of the prepared cementitious materials and the relevant cementation mechanism were investigated by means of mechanical test, XRD, FT-IR, MAS NMR and SEM-EDS. Results show that the slag content has significant effect on the pastes fluidity and compressive strength, while the NH module only has significant effect on the compressive strength of AACGS cementitious materials. Higher early strength may be produced for AACGS materials, for instance, when the slag content exceeds 20%, the 28 d strength is also higher than that of P. O42.5 pure cement. With the increase of slag content, the higher polymerization degree emerged for the silicate structure in the hydration products, which is consistent with the measured results of compressive strength. Since that Ca²⁺ has a stronger drive adsorption force than Na⁺, therewith the increase of slag content from 0 to 50% may result in denser matrix, of which the C-(A)-S-H in the microstructure has higher Ca/Si ratio but lower Al/Si ratio. The disordered network structure of AACGS, which consists of C-A-S-H gels and C-S-H gels and N-A-S-H gels, has good compatibility.