Abstract: In the aging management of nuclear power plants, the structural integrity of the seawater system pipeline is essential for the safe operation. In order to realize the daily rapid inspection of pipeline, a quantitative evaluation method based on low-frequency electromagnetic detection (LFED) technology, combined with borescope inspection for open validation was proposed in this paper. Practical inspection of operational seawater pipelines in a nuclear power plant demonstrated the effectiveness of this LFED-based detection and evaluation approach. The results indicate that two suspected signals were detected via LFED. Subsequent borescope measurements revealed their wall thicknesses of 6.58 mm and 3.25 mm, corresponding to actual wall thickness losses of 17.8% and 59.3%, respectively. The LFED method showed an error margin of 5%, confirming its strong defect detection capability and quantitative evaluation accuracy. The low-frequency electromagnetic testing technology can be applied to online pipeline corrosion monitoring and condition assessment, providing effective technical support for the structural integrity of metallic pipelines in nuclear power plants.