A cooler system (100; 100'), which cooler system comprises: a plurality of cooler units ; a condensed water flow path , an aqueous medium in the condensed water flow path sequentially flowing through each of the cooler units; a cooling water flow path , an aqueous medium in the cooling water flow path sequentially flowing through each of the cooler units in a flow direction opposite to a flow direction of the aqueous medium in the condensed water flow path; and a plurality of current sensors used for detecting working currents (I1, I2, I3) of the cooler units; water temperature sensors used for detecting an entering water temperature Tenter and a leaving water temperature Tleave of the cooling water flow path; a virtual temperature sensor arranged between each of the cooler units and used for acquiring a middle water temperature (Tmid1, Tmid2) on the cooling water flow path between each of the cooler units, wherein acquiring the middle water temperature is based on a ratio relation among a ratio of temperature differences between an entering water temperature and a leaving water temperature of each of the cooler units, a ratio of rated refrigeration capacities (C1, C2, C3) of the cooler units, a ratio of the working currents (I1, 12, 13) of the cooler units and a correction factor; and a controller used for setting a middle water temperature set value (Tmid1_set, Tmid2_set) and a leaving water temperature set value (Tleave_set) and capable of adjusting a refrigeration capacity of the cooler units to enable the middle water temperature to be close or equal to the middle water temperature set value and to enable the leaving water temperature to be close or equal to the leaving water temperature set value. a plurality of cooler units ; a condensed water flow path , an aqueous medium in the condensed water flow path sequentially flowing through each of the cooler units; a cooling water flow path , an aqueous medium in the cooling water flow path sequentially flowing through each of the cooler units in a flow direction opposite to a flow direction of the aqueous medium in the condensed water flow path; and a plurality of current sensors used for detecting working currents (I1, I2, I3) of the cooler units; water temperature sensors used for detecting an entering water temperature Tenter and a leaving water temperature Tleave of the cooling water flow path; a virtual temperature sensor arranged between each of the cooler units and used for acquiring a middle water temperature (Tmid1, Tmid2) on the cooling water flow path between each of the cooler units, wherein acquiring the middle water temperature is based on a ratio relation among a ratio of temperature differences between an entering water temperature and a leaving water temperature of each of the cooler units, a ratio of rated refrigeration capacities (C1, C2, C3) of the cooler units, a ratio of the working currents (I1, 12, 13) of the cooler units and a correction factor; and a controller used for setting a middle water temperature set value (Tmid1_set, Tmid2_set) and a leaving water temperature set value (Tleave_set) and capable of adjusting a refrigeration capacity of the cooler units to enable the middle water temperature to be close or equal to the middle water temperature set value and to enable the leaving water temperature to be close or equal to the leaving water temperature set value. <EOS>