The invention relates to the technical field of flow direction control, and provides a flow direction control valve and a heat pump system. The flow direction control valve comprises a valve body, a first cavity and a second cavity which are communicated with each other are formed in the valve body, and the valve body is provided with a third connector, a fourth connector, a first connector and a second connector, the outer piston is in sliding connection with the first cavity, a gap communicated with the second cavity is formed in the outer piston, the other end of the outer piston is in sealed connection with the first cavity, and a pressure balance hole communicated with the second cavity is formed in one end of the outer piston; and the inner piston is in sealed sliding connection with the mounting cavity in the outer piston, and the inner piston can be switched between a compression state and an initial state. The side piston is connected with the second cavity in a sealed and sliding mode, and the side piston can be switched between a compression state and an initial state. According to the flow direction control valve, switching control over the flow direction of fluid such as refrigerants is achieved, the structure is simple, and cost is reduced.