Abstract: The main function of the important service water (SEC) system in nuclear power plants is to receive the nuclear island heat load collected by the equipment cooling water system (RRI) through a plate heat exchanger under any operating conditions, and transport the nuclear island heat load to the final hot well seawater. The article briefly introduces the event of a nuclear power plant unit experiencing a sudden drop in the heat transfer coefficient of its plate heat exchanger during a major overhaul, which once approached the minimum limit requirement. Subsequently, the possible causes of this phenomenon were analyzed and investigated, and it was determined that it was caused by the reverse installation of the plate heat exchanger plates. After disassembling and reassembling the plate heat exchanger, the phenomenon disappeared. Finally, several optimization suggestions were formulated to address this phenomenon.