During the commissioning period of Unit 5 of Tianwan Nuclear Power Plant，the high-pressure heater frequentlyexperienced abnormal water level rise and triggered protection disconnection when it is put back into operation after beingdisconnected. To solve this problem，this paper analyzes the system structure and hydrophobic control logic，uses the enumeration elimination method to investigate the cause of the fault，and confirms that hydrophobic flash evaporation is the fundamental cause of the false high water level. Based on this，measures such as optimizing operation，adjusting drainage settings，and improving monitoring and control logic are proposed. Through on-site verification，this measure can effectively avoid abnormal fluctuations in water level，ensure smooth operation of the heater，and provide certain reference for optimizing the operation of high-pressure heaters in similar units.