Abstract: The filling technology is an important part of green mining. The low-carbon environmental-friendly filling materials with low costs in research and development and reliable performance are critical for the development of filling technologies. The coal gangue (CG) and coal-series metakaolin (MK) were adopted as the raw materials to prepare coal-series solid-waste-base green filling materials, and the effect of the mixing proportion and alkali activator on the strength and fluidity of filling materials was discussed. In combination with the X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric (TG) analysis and scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM-EDS) and other characterization measures, the filling strength development mechanism was revealed. Considering the strength, fluidity and environmental indexes, the mixing proportion of filling materials was optimized. As indicated by the research results, in the green filling material system, the coal-series metakaolin generates gelatinization effects through alkali-activated hydration reaction. The system strength is increased in a linear mode with the increase of metakaolin. The finely-grounded coal gangue is used as the inert filler that coordinates with Na₂SiO₃ to improve fluidity. The main hydration products of this filling material are N ― A ― S ― H and zeolites. Right after Si ― O ― Si depolymerization, the tetrahedral Al ― O bond takes the place of Si ― O bond, resulting in the change of (SiO₄)⁴⁻ into (AlO₄)⁴⁻. Through further polymerization, the Si ― O ― Al group is generated. When the ratio between Na₂SiO₃ and NaOH is 1∶1 in the alkali activator, the polymerization degree is the highest. The hydration products are filled into the pores of coal gangue particles, leading to dense matrix and stronger filling materials. As recommended based on the comprehensive index evaluation, the mixing ratio between metakaolin and coal gangue shall be 3∶7, which does not only meet the requirements of strength and fluidity but also provides the carbon emission index as low as 0.257. This research provides new approaches for developing the low-carbon environmental-friendly filling materials with low costs and reliable performance, and is highly practical and economically efficient.