Abstract: The circulating cooling water system（hereinafter referred to as RCW system） and chilled water system using inhibitor is different in CNNO NPP3. The chemical analysis showed that the corrosion inhibitor by nitrite is frozen water leaking into the hydrazine/lithium hydroxide as RCW inhibitors will make the iron in water content increased significantly. In this paper, by investigating the rising of the nitrite ion in RCW system, the impact of chilled water leaking into RCW system is evaluated, and the corrective measures to be taken are analyzed. Ccombined with the laboratory and field tests, it is concluded that although the corrosion rate of metal materials resulting from the nitrite ion in RCW system exceeded that in normal operation, but it is within the acceptable range. At this stage, the corrosion can be mitigated by natural water feedand drainage to reduce the impurity ions. In view of long term operation, it is recommended to replace the inhibitor of the chilled water at the appropriate time, thus to solve the impact of chilled water leaking into RCW system completely.