The invention discloses a water chiller unit for the air-conditioner refrigeration field. The water chiller unit comprises a compressor, a condenser, a liquid reservoir, a drying filter, a liquid mirror, an electronic expansion valve, a first casing type evaporator, a second casing type evaporator, an adjusting valve, a cooling water pump, a first chilled water pump, a second chilled water pump, a three-way valve and refrigerant. The outlet end of the electronic expansion valve is connected with the first inlet end of the first casing type evaporator, the first outlet end of the first casing type evaporator, the first inlet end of the second casing type evaporator and the outlet end of the electronic expansion valve are connected with three orifices of the three-way valve respectively. The water chiller unit solves the problems that an existing one-off cooling and dehumidifying air conditioner is low in evaporation temperature, air supply humidity is difficult to accurately control, energy efficiency ratio of a refrigerating unit is hard to increase. Meanwhile, the invention further provides an operating method of the water chiller unit, and two kinds of chilled water different in temperature are effectively provided. Further, the invention provides the refrigerant. Performance of a heat and humidity segment treatment system can be improved.