The invention discloses a multifunctional drying and dehumidifying device and a drying and dehumidifying method thereof. The drying and dehumidifying device comprises a heat pump assembly, a drying assembly and a controller. The heat pump assembly comprises a compressor, a four-way valve, a condenser, a one-way valve, a liquid accumulator, a filter, a thermal expansion valve, a gas-liquid separator, an outdoor evaporator, a first dehumidification evaporator and a second dehumidification evaporator. The evaporators are each provided with an electromagnetic valve. The drying assembly comprises a dryer shell body, a circulating draught fan, a moisture exhausting draught fan, an air heat exchanger and an outdoor draught fan. According to the structural design, the drying and dehumidifying device has the advantages of being novel in structural design, diversified in function and good in energy conservation and environmental protection effect. In addition, the drying and dehumidifying method comprises the following operating modes of a ventilation mode, a heating mode, a constant-temperature dehumidifying mode, a cooling dehumidifying mode, a heating moisture exhausting mode and a constant-temperature dehumidifying moisture exhausting mode. According to the design of the operating modes of the drying and dehumidifying method, a plurality of functions can be realized, and the good effects of energy conservation and environmental protection are achieved.