The performance of supercritical multi-thermal fluid injection in heavy oil reservoirs is markedly superior to that of steam flooding. However, the underground seepage law of injecting supercritical multi-thermal fluid is not yet clear. Therefore, the "high-temperature and high-pressure steady-state method" was proposed to test the oil-water and oil-gas relative permeability curves at different temperatures. The viscosity of produced heavy oil and the contact angle between oil sand and water at different temperatures were tested, and finally, combined with the oil-water, oil-gas relative permeability and Stone-Ⅱ prediction model, the isoperms of oil phase relative permeability in different hot areas during three-phase seepage were obtained. The results show that after the action of supercritical water on heavy oil, the measured viscosity of produced heavy oil decreased by 31.03% at 50 ℃ compared to steam, and the contact angle between oil sand and water decreased from 139.5° to 100.9°, indicating that the wetting properties of the oil sand develop towards a water-wet direction. Compared with the relative permeability of oil phase, the relative permeability of water phase is very small, and the characteristic value of oil-water relative permeability curve changes gradually and then suddenly at supercritical temperature. The relative permeability of oil-gas increases gradually with the increase of temperature. In the isoperms of oil phase relative permeability, the area of the oil flow zone expanded as temperatures rose, and under supercritical conditions, the flow zone area grew to 54.59%, highlighting a significant enhancement in oil-phase flow capacity. The research results of this paper can provide theoretical basis for the seepage mechanism and numerical simulation of supercritical multi-thermal fluid injected in heavy oil reservoirs.