The invention relates to an air source heat pump water heater. The air source heat pump water heater comprises a compressor, a four-way valve, a gas cooler, a throttle valve, an evaporator, a gas-liquid separator, a defrosting solenoid valve, a fan and a heater, wherein the compressor, the four-way valve, the gas cooler, the throttle valve, the evaporator and the gas-liquid separator form a circulation circuit, the defrosting solenoid valve and the throttle valve are connected in parallel, the heater is arranged on the outer surface of the evaporator and the fan capable of acting wind on the evaporator surface is located on one side of the evaporator, and a temperature detection device is arranged on a coil pipe of the evaporator, and the defrosting solenoid valve, the fan and the heater are connected with a controller. According to the air source heat pump water heater, purposeful defrosting work can be performed for frost with different thicknesses through an electric heater, the defrosting solenoid valve and the four-way valve, so that it is ensured that the air source heat pump water heater does not reduce the energy consumption while working, and the defrosting work can be realized step by step in an early stage of frosting.