The main feed water flow for nuclear power units is measured by Venturi tubes. These use the principle of differential pressure to act as throttling devices. Over the course of their operational life, the passage of medium- and high-speed fluids can erode Venturi tubes, undermining the measurement accuracy of the differential pressure and flow. Venturi tubes play a vital role in nuclear power units, participating in the regulation of the water level in steam generator control systems and protecting reactor signals. Thus, the accuracy of their signal measurements directly affects the safe and stable operation of nuclear power units. This paper analyzes statistical data regarding the main feed water flow measurements from 26 nuclear power units. Combined with the working principles of Venturi tubes, it establishes the mechanism governing erosion in Venturi tubes and its characteristics over long-term operation. A multi-level assessment method for Venturi tube erosion then is proposed that can evaluate the accuracy of flow measurements in terms of degradation, failure and replacement. On the basis of this method, accurate corrections of the differential pressure can be implemented or the tubes replaced. The method was applied in nuclear power units and its effectiveness verified. It can therefore provide a reliable basis for improvements in the safety of nuclear power unit operation and serve as a reference for other chemical and thermal power fields where Venturi tubes are used for measurement and regulation.