Abstract: The synergistic CO₂ sequestration via solid waste backfilling in goafs can simultaneously address the issues of CO₂ emissions, accumulation of coal-based solid wastes, and safety hazards in goafs under China’s coal-dominated energy structure. In this study, a modified magnesium-coal-based all-solid-waste carbon-sequestering backfill material (MFCC, prepared from modified magnesium slag (MMS), fly ash (FA), coal gangue (CG), and coal gasification slag (CGS)) was fabricated. The fluidity of the fresh slurry was characterized using the mini slump test, and its carbonation curing performance was investigated via uniaxial compressive strength (UCS), carbonation depth (CD), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry-differential thermogravimetry (TG-DTG), and computed tomography (CT) tests, aiming to achieve the synergistic goals of high-value utilization of solid wastes and CO₂ sequestration. The results indicate that the fresh MFCC slurry exhibits excellent fluidity with a mini slump ranging from 121.5 to 135 mm. The fluidity increases with the rise in CGS content, which fully meets the requirements for industrial pipeline pumping. During the carbonation curing process, the UCS of the material increases continuously with the extension of curing age, with the 28-d UCS ranging from 7.36 to 8.71 MPa, which fully meets the strength design requirements for coal mine backfilling engineering. Microscopic analyses reveal that the filling and cementation effects of hydration and carbonation products on pores render the material’s microstructure denser, significantly reducing pore volume and connectivity, which is the key reason for the strength improvement. After 28 d of carbonation curing, when the CGS content is 20wt%, the UCS reaches a maximum value of 8.71 MPa, and the CO₂ uptake also attains a peak of 13.94%. In summary, after carbonation curing, the MFCC material not only exhibits excellent mechanical properties but also enables the simultaneous realization of resource utilization of solid wastes and efficient CO₂ sequestration, thus holding broad application prospects in backfilling engineering.