The invention discloses a method for treating cooling water in generators of 600 MW and above. The method comprises the following steps of: (1) replacing a bypass small mixing bed in a cooling water tank in a to-be-treated generator of 600 MW and above with an electric demineralizer, wherein a metering pump and a pH gauge are connected in parallel to the input port of the electric demineralizer; the pH gauge is further connected to an electrical conductivity gauge; and another pH gauge and another electrical conductivity gauge are connected in series to the output port of the electric demineralizer; (2) adjusting the flow rate of the cooling water and determining drainage time; (3) adjusting the pH value of the cooling water at the input port to be consistent with that at the output port;(4) collecting high deviation of the electrical conductivity of cooling water in the generator; and (5) stopping or adding in sodium hydroxide solution until pH value is in the range of 8.0-9.0 and consistent with that at the output port. The method has the following advantages that the quality of the cooling water measures up to national regulations; the service life of the generator can be prolonged by 10 years; and with unattended operation, the labor and materials are saved, and the method is efficient and convenient.