Abstract: The local thinning of the high-pressure heater system of a nuclear power plant occurred during the service process. The reasons for the thinning of the wall thickness of the trap were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope analysis and X-ray diffraction, and the flow pattern of the trap channel was simulated. The results show that the erosion thinning of high velocity medium and the fluid accelerated corrosion thinning were the direct causes of the thinning perforation failure of the trap. The decarburization of the inner surface of the valve body led to the decrease of the hardness, erosion resistance and corrosion resistance of the material, which accelerated the wall thickness reduction of the valve body.