The expansion valve is an important part in the refrigeration system, which is generally installed between the liquid storage tank and the evaporator. By throttling, the expansion valve turns the liquid refrigerant with medium temperature and high pressure into wet steam with low temperature and low pressure, and then the refrigerant absorbs heat in the evaporator to achieve the refrigeration effect. The expansion valve controls the valve flow by changing the superheat at the end of the evaporator, so as to prevent the underutilization of the evaporator area and cylinder knocking.