Abstract: During CO? injection fracturing or displacement-enhanced coalbed methane extraction, the mechanism by which sequential supercritical CO? (SCCO?)-liquid CO? (LCO?) treatment affects coal’s microstructure and mechanical properties remains unclear. To address this, anthracite was used as the research object, with sequential SCCO?-LCO? treatment experiments conducted combined with nanoindentation and optical 3D topography analysis to systematically investigate the modification effects. Microstructurally, SCCO? treatment increased coal’s horizontal 2D roughness by 2.37%~42.60% and most 3D roughness parameters (arithmetic mean height: 0.19%~217.50%; interfacial area expansion ratio: 2.90%~116.58%; 3D root mean square height: 3.14%~204.17%), whereas subsequent LCO? treatment reduced vertical 2D roughness by -2.41%~-67.89% and most 3D roughness parameters (arithmetic mean height: 0.46%~56.18%; interfacial area expansion ratio: 0.32%~40.04%; 3D root mean square height: 2.58%~52.20%). Micromechanically, SCCO? treatment decreased coal’s average elastic modulus by 5.38% and hardness by 8.5%, with the elastic energy ratio falling from ~82% to ~77%; subsequent LCO? treatment caused a further 5.19% decrease in elastic modulus and 1.17% in hardness (cumulative weakening) without significant change in elastic energy ratio. Normalized correlation analysis indicated that the convergence/dispersion of elastic modulus and hardness data reflects distinct CO? phase mechanisms: SCCO? enhanced coal heterogeneity, while LCO? did not exacerbate it. This study reveals the differential modification mechanism of sequential SCCO?-LCO? action on coal structure at the microscale, providing theoretical and experimental support for optimizing CO? injection to improve sequestration safety and coalbed methane recovery.