Under the background of the “dual carbon” goals, exploring new pathways for industrial restructuring, reducing carbon emissions, improving energy efficiency, and developing new high-quality productivity for CO₂ high-energy utilization has become a research focus. To determine the mechanism of CO₂ phase-change-induced cracking and expand its engineering applications, this study investigates the safety and environmental advantages of CO₂ based on its multiphase characteristics and energy-utilization potential. A numerical gas-water coupling model is established. Using CO₂ at 25 ℃ as the research object, a simulation scheme is designed to examine different water temperatures as heat-carrying fluids, thereby revealing the CO₂-water heat-transfer mechanism under hydrothermal-CO₂ coupling. Furthermore, the heat and mass transfer characteristics of two-phase flow are analyzed by a CO₂-water enhanced tube-type heat-exchanger test. The results show that as the hydrothermal-fluid temperature increases, the CO₂ heating rate increases proportionally: for every 1 ℃ increase in water temperature, CO₂ temperature increases by 0.9 ℃. The water-stream temperature gradually decreases as CO₂ absorbs heat, and the dissipation temperature of the hydrothermal fluid is proportional to the CO₂ absorption temperature. The heat-transfer coefficient of the hydrothermal fluid increases from 1 790 W/(m²·K) to 2 090 W/(m²·K) and continues to rise with higher initial water temperatures. The heat transfer coefficient of CO₂ is positively correlated with that of the hydrothermal fluids. The CO₂ phase-change heat-absorption temperature shows an exponential growth trend with increasing water temperature. The maximum internal CO₂ pressure increases from 131 MPa to 199 MPa, undergoing sequential stages: thermal expansion of liquid CO₂, thermal expansion of gaseous CO₂, phase-change energization, and pressure stabilization. CO₂ heat absorption is positively correlated with the initial water temperature, and the thermal power of the water-flow heat source increases with rising water temperature. The effectiveness of supercritical CO₂-water convective heat transfer is verified through the establishment of correlation equations and experimental analysis. This research provides significance theoretical and engineering insights for energy-conversion applications under non-uniform heat-flow conditions, such as photothermal systems, boilers, and CO₂ fracturing and storage.