The invention provides a water heater. The water heater comprises an air source heat pump heating system and a water storage tank. The air source heat pump heating system comprises a water flow heat exchanger, an air heat exchanger and a working medium flow direction switching device which are connected through a refrigerating working medium pipeline, and moreover the air source heat pump heating system is configured to be used for acquiring a defrosting starting signal and switching the operation condition of the working medium flow direction switching device to be in the condition that the water flow heat exchanger serves as an evaporator and the air heat exchanger serves as a condenser so as to remove frost attached to the air heat exchanger. The water storage tank is connected with a water flow pipeline of the water flow heat exchanger and is configured to be used for supplying water to the water flow heat exchanger in the process of removing the frost on the air heat exchanger so as to carry away the cold energy of the water flow heat exchanger. Accordingly, the defrosting efficiency is improved, and the effect that the water heater still works normally in a cold working environment in winter can be ensured.