It is difficult for conventional air-source heat pumps (ASHP) to produce high-temperature hot water in subarctic regions with low ambient temperatures. When the ambient temperature is -30 ℃, the heating performance of a single-stage ASHP is significantly attenuated, making it difficult to maintain an effective heating cycle. In order to solve this problem, a thermodynamic analysis model is developed for a cascade ASHP water heating system capable of producing 80 ℃ high-temperature hot water in a -30 ℃ low-temperature environment, which can be used for heating in the severe cold zone. In this paper, four high-temperature stage refrigerants (R134a, R407C, R236ea and R1234ze (Z)) are each paired with a low-temperature stage refrigerant (R32), and they are compared using performance metrics including the heating COP, compressor parameters, exergy losses and exergy efficiency of the system. The results show that when the ambient temperature is -30 ℃, the system with R1234ze (Z)/R32 refrigerant combination has the best heating effect. The changes in the maximum COP value, optimal intermediate temperature, optimal mass flow ratio, compressor parameters, exergy losses and exergy efficiency of the cascade ASHP water heater are also studied when the ambient temperature increases from -30 ℃ to 30 ℃, and their relationships with the ambient temperature are analysed.