Abstract: To clarify the synergistic mechanism between CO₂ and crude oil-formation water-rock in the reservoir during CO₂-assisted steam flooding, a series of experiments including PVT experiments, solid phase deposition experiments, high-temperature and high-pressure interfacial tension tests, water-rock reaction experiments, and oil-gas-water three-phase seepage experiments were conducted to systematically investigate the action mechanism of CO₂ under steam flooding conditions. The results show that CO₂ exhibits a synergistic effect of dissolution-swelling-viscosity reduction-demulsification in water-bearing crude oil. With the increase of pressure, the solubility of CO₂ increases significantly, the crude oil volume expansion coefficient reaches up to 1.11, the viscosity reduction rate exceeds 48%, and efficient demulsification capacity is still maintained under low water-cut conditions. The extraction of C₅—C₂₅ components from crude oil by CO₂ dominates, forming a competitive relationship with solid phase deposition, and the deposition amount is <0.05 kg/m³, indicating good engineering feasibility. The acid reaction of CO₂-formation water-rock induces the rock wettability to change from strongly oil-wet to strongly water-wet, which significantly improves the oil-water two-phase seepage. High-temperature relative permeability experiments confirm that the addition of CO₂ reduces the residual oil saturation by 11.1 percentage points, greatly enhancing the oil displacement efficiency. This study provides systematic experimental basis and theoretical support for the mechanism understanding and field application of CO₂-assisted steam flooding.