Abstract: Driven by energy structural transformation and the "dual carbon" goals, underground coal gasification (UCG) has drawn significant attention as a key technology for clean coal utilization. Research has shown that UCG, with its unique high−permeability fracture network, strong adsorption capacity of residual coke, and high−temperature geological environment, has significant potential as an emerging CO₂ geological storage medium. This study systematically investigated the formation mechanism and spatial fixation of the combustion cavity, revealing the multiphase migration characteristics of CO₂ within the cavity: early on, driven by buoyancy, it accumulates beneath the caprock to form structural traps. Locally, it is physically adsorbed by residual coke through diffusion, and over time, it undergoes mineralization reactions with alkaline ash residues to form stable carbonates. To assess storage safety, this study employed Thermo−Hydro−Mechanical−Chemical (THMC) multi−field coupled simulation simulation technology to characterize the rock mass stress distortion and fracture propagation induced by CO₂ migration. Furthermore, it identified caprock defects, injection pressure threshold, and cavity size as key control parameters influencing long−term stability. Key conclusions indicate that Coupling with carbon capture and storage (CCS) can significantly reduce the carbon footprint of UCG, offering a new paradigm for low−carbon coal development. However, current challenges include reduced gas injection efficiency due to heterogeneous residue distribution, the lack of a mechanism for thermal damage associated with CO₂−surrounding rock formation, and inaccurate predictions of further caprock fracture formation. Future research should focus on developing multiphase transport−mineralization kinetic reaction models, investigating dynamic fracture monitoring, and optimizing "annular backfill structures" to advance the potential of this technology toward engineering applications.