The invention discloses a low refrigerant charging amount heat pump device and a control method thereof. The device comprises a compressor, a compressor refrigerant gas outlet is sequentially connected with an indoor heat exchanger, a throttling device and an outdoor heat exchanger through a four-way reversing valve, an outlet of the outdoor heat exchanger is connected with an inlet of a liquid storage device through the four-way reversing valve, an outlet of the liquid storage device is connected with an inlet of the compressor, and the device further comprises a bypass pipeline; an inlet of the bypass pipeline is located between the compressor and the four-way reversing valve, an outlet of the bypass pipeline is located between the four-way reversing valve and the liquid storage device or located on the side or bottom of a shell of the liquid storage device, and a bypass electromagnetic valve is located on the bypass pipeline; under the heating condition, when the device stably runs, a refrigerant exhausted out of the compressor flows into the indoor heat exchanger through the four-way reversing valve and then flows back to the compressor through the throttling device, the outdoor heat exchanger, the four-way reversing valve and the liquid storage device, and a heating cycle is formed; and under the refrigerating condition, stable running is opposite to the heating cycle flow path. The control method of the device is further provided. In the cold starting process, the problem that a large number of refrigerants are gathered in the liquid storage device and the compressor, and consequently the pressure of an air inlet of the compressor is too low is solved.