A comparative experiment prototype of an air-source heat pump water heater（ASHPWH）system was established for the comparative study of instantaneous heating and circulation heating modes. At ambient temperature of (19±0.5)℃，176 kg of water was heated from 20℃ to 55℃ with the two modes respectively. The average COP of the instantaneous heating mode is 24% higher than that of the circulation heating mode，and the average condensation heating power is also increased by 20%. The instantaneous heating mode not only has a higher COP，but also has a higher condensation heating power，resulting in a lower power consumption and a shorter heating time. At the same time，the condensation pressure，the compression ratio，the maximum outlet temperature of the compressor and other important parameters with the instantaneous heating mode are superior to those with the circulation heating mode. The present study shows that ASHPWH system has superior thermal performance in the instantaneous heating mode than in the circulation heating mode.