The invention discloses a quasi-two-stage compression ultralow temperature air source heat pump water heater with an economizer, which mainly comprises a gas-supply enthalpy-rise compressor, an air-cooling evaporator, a water side heat exchanger, a gas-liquid separator, an economizer, a four-way valve, an electronic expansion valve, a drying and filtering device, a liquid accumulator, an electromagnetic valve, a thermal expansion valve and a one-way valve. The invention is characterized by adopting a quasi-two-stage compression technology, optimizing the structure of the evaporator, widening spaces between fins of the evaporator and using the reverse defrosting technology. Proved by system property test experiments and engineering practices, the quasi-two-stage compression ultralow temperature air source heat pump water heater with the economizer can improve energy efficiency ratio by 10% at the ambient temperature of 0 DEG C, by 15% at the ambient temperature of -5 DEG C and by about 30% at the ambient temperature of -10 DEG C and can still work normally to supply hot water even in ultralower ambient temperature of -15 DEG C; and in a frosting temperature range from 2 DEG C to -2 DEG C, the quasi-two-stage compression ultralow temperature air source heat pump water heater with the economizer can defrost quickly, evenly and thoroughly, thereby effectively solving the problem of frosting in winter in humid and cold area in the south of china.