The research object is an office building in Suzhou which is equipped with a radiation and fresh air system with separate temperature and humidity control based on the form of energy storage tanks. The models of the direct supply form of air source heat pumps and the radiation and fresh air system with separate temperature and humidity control based on the form of energy storage tanks are established by using Trnsys. These models are validated using measured data from the actual systems. A comparative analysis of simulation results between systems with and without energy storage water tank is conducted. The findings indicate that integrating 0.15 m³, 0.2 m³, 0.5 m³, and 1 m3 energy storage water tanks into direct supply systems reduces fluctuations in main unit outlet water temperature, decreasing start-stop frequencies by 57.8%, 60.9%, 71.1% and 77.2% respectively. However, energy consumption slightly increases by 2.1%, 2%, 2.4% and 3.1%, respectively. The coefficient of performance of the heat pump unit remains unchanged regardless of water tank volume or system configuration.