The invention relates to an easy-defrosting multifunctional heat pump water heater and solves a problem that a single air source and water source heat pump has to have both cooling and heating applications and meet different heat needs of all the year around and a frosting problem. The water heater comprises a refrigeration compressor, a fin-type air-cooled heat exchanger with a fan, and double-pipe heat exchangers A and B. An inlet side of the refrigeration compressor is serially connected with a gas-liquid separator. An outlet side of the refrigeration compressor is serially connected with a four-way valve A, a four-way valve B and the fin-type air-cooled heat exchanger with the fan and then forms a circulation loop through a one-way valve, a liquid vessel, a filter drier, level glasses, a three-way pipe and a four-way pipe. The device realizes four separated circulation systems, and can well realize full cooling and heating and supply heat meeting needs through different heat sources. The device has four functions of cooling, heating, making domestic hot water, and cooling and making domestic hot water. A process designed for the device can reasonably and reliably remove frost during operation in winter.