Accurate measurement of water cut in produced fluid is crucial for effective oilfield production monitoring and reservoir management. However, discrepancies often arise between the claimed performance specifications of commercial online water cut instruments and their actual measurement accuracy under real field conditions. This study aims to develop a comprehensive evaluation model for water cut measurement methods, specifically tailored to the unique distribution characteristics of produced fluid in oilfields, thereby providing a scientific basis for instrument selection and performance optimization. The research integrates field data of water cut and salinity distribution from representative oilfield blocks and systematically analyzes the measurement principles and error characteristics of various online water cut instruments. A full-range evaluation model is constructed, incorporating a weighted assessment approach based on the statistical distribution of water cut, with particular focus on high-water-cut stages commonly encountered in mature oilfield production. The findings reveal that, despite the nominal 0–100% measurement range claimed by most instruments, significant measurement deviations are observed under actual field conditions, especially in high water cut regimes. The proposed evaluation model effectively quantifies these deviations and offers an objective framework for assessing the performance of different instruments in varying operational scenarios. This study provides a robust reference for the selection, installation, and management of online water cut instruments in oilfield applications, improving the accuracy and reliability of produced fluid measurement and supporting enhanced decision-making in oilfield production operations.