One of the inherent limitations of air source heat pump water heaters is their reduced performance when the external temperature is low. In order to investigate the impact of water supply temperature on system heating in low temperature conditions, a test bench for an air source heat pump water heater with a medium pressure air supply was constructed. The findings illustrate that within the two low temperature settings of -7°C and -15°C, the heating capacity of the system initially increases with the rise in water supply temperature, reaching a peak at 39°C. However, this effect is not sustained as the temperature of the water supply continues to rise. Concurrently, the system's heating capacity reaches its maximum at a water supply temperature of 39°C. At this temperature, the heating capacity is 45.8% lower than at -7°C, the compressor power is 42.1% lower, and the system coefficient of performance is 7.2% lower.