The invention provides a four-way valve, an air conditioning unit and a control method. The four-way valve comprises a main valve body, a valve box is arranged in the main valve body, a valve elementis arranged in the valve box, and the main valve body is further provided with a communication port D, a communication port E, a communication port S and a communication port C which communicate withthe valve box. According to the four-way valve, the air conditioning unit and the control method, the four-way valve is changed to have a third position, the communication port E can be closed, and thus a shell pass of an evaporator is closed, refrigerants are effectively prevented from entering a gas-liquid separator through the communication port E under the action of inertia, the refrigerants are closed in the shell pass and carry out heat exchange with water in a heat exchange pipe in the evaporator, the water is prevented from freezing in the heat exchange pipe and the heat exchange pipeis prevented from frost splitting under the low temperature working condition.