A preliminary experimental investigation was carried out to analyze the thermal characteristics of a transcritical CO2 air source heat pump unit with thermal energy storage for residential heating. The primary impact factors such as discharge pressure, ambient temperature, supply/return water temperature were considered. Also, a performance comparison of the combined system with the CO2 air source heat pump unit without thermal energy storage was presented. It was found that the maximum coefficient of performance (COP) of the CO2 air source heat pump unit decreased by 18.8%, and the optimum discharge pressure and the throttle loss increased by 14.7% and 47.5%, respectively, as the water temperature at the inlet of the gas cooler increased from 35 to 45 °C. Additionally, the results also indicated that the COP of the combined system was 17% higher than that of the CO2 air source heat pump unit. However, it was worth noted that the water flow rate should be adjusted according to the supply water temperature in the practical application so as to meet the thermal comfort for user. The experimental results demonstrated that thermal energy storage as a sub-cooler is a promising technology for reducing the throttle loss and improving the COP of the combined system, and is helpful to promote the CO2 air source heat pump application for space heating in cold regions.