Abstract: The design life of the heat exchanger of the recirculation cooling water system in Third Qinshan Nuclear Power Plant was 40 years, and they are the nuclear safety related equipment. After three years of use, a large number of erosion characteristics have appeared in the tube sheet and heat exchanger tube. Through the analysis, it was found that the operation parameters of the system were higher than the design value, and the material selection, equipment selection and structural design were not suitable for the matching of the environmental characteristics of the sea water, which was the primary cause for the severe erosion of the material. And the mechanical characteristics of the local flow field were changed by the peeling off of the rubber lining, which was the direct cause for the sudden serious damage to the equipment. By completing the work such as the system transformation, tube change, tube plate repair and protection, and re-lining, the long term reliability of the equipment has been restored and improved.