A three-dimensional (3D) numerical model of the double-tubes heat exchanger was established by using fluent16.0 in this study. The supercritical flow and heat transfer characteristics of R744 and its two azeotropic blends in the gas cooler and their performance in the single-stage compression heat pump were investigated. The results showed that the peak value of the heat transfer capabilities of R744 was the highest, and that of R744/R170 was the lowest. With the increase in the pressure, the peak value of the heat transfer capabilities decreased, and the bulk temperature corresponding to the peak value of the heat transfer capabilities also increased. Moreover, the temperature of R744/R170 at the same position near the outlet of the tube section was lower under the different working conditions, which would be conducive in increasing the coefficient of performance (COP) of the system. Lastly, when R744 and its two azeotropic blends were employed in the single-stage compression heat pump system, the difference of COP between R744/R170 and R744 decreased with the increase of the outlet temperature in the gas cooler, and the difference of COP between R744/R41 and R744 increased.