The utility model discloses circulating cooling water system moisturizing device, including storage water tank, main filter and three -way valve, be equipped with circulation water -supply pathway between the delivery port of storage water tank and the water inlet of storage water tank, on circulation water -supply pathway was located to main filter and three -way valve, the first delivery port of three -way valve was linked together with circulating cooling water system, lay in on the circulation water -supply pathway to be equipped with first valve, circulating pump and second valve between delivery port and the main filter of storage water tank in proper order. The utility model discloses a water of circulating pump in with the storage water tank is taken out to circulation water -supply pathway, water is at circulation water -supply pathway internal flow measured time, purified to the pure water through main filter, later get into circulating cooling water system through the three -way valve, automatically, accomplish moisturizing work, the flow of two exports of distribution can be adjusted to the three -way valve, reach the mesh of accurate control moisturizing volume, satisfy different on -the -spot operation requirements, unnecessary water is got back to in the storage water tank from the second delivery port of three -way valve again, can not cause the waste.