For coastal estuary water quality monitoring environments, traditional conductivity sensors suffer from issues such as bulky size and susceptibility to corrosion. This paper proposes a non-contact seawater conductivity measurement method based on single-coil sweep-frequency resonant impedance measurement. A coil equivalent circuit model in seawater environments was established, with in-depth analysis of the mechanism by which seawater eddy current losses affect system resonance characteristics. It elucidates the linear mapping relationship between resonant equivalent impedance and seawater conductivity under resonant conditions. Finite element simulation was employed to perform linear fitting on simulated data, validating the accuracy of theoretical derivations. Building on this, a sweep-frequency-based conductivity measurement system was constructed, achieving precise extraction of resonant point impedance. Experimental results demonstrate that in low-conductivity environments (saltwater intrusion), this method maintains consistent high measurement sensitivity, with a maximum fitting error of merely 0.0417 mS/cm. Compared to existing research, the proposed approach significantly enhances detection precision for subtle conductivity variations while improving anti-contamination capabilities. Furthermore, this method enables pre-calculation of fitting parameters via simulation software, thereby reducing human and material resources required for sensor calibration and optimizing sensor fabrication processes. It offers a novel solution for estuary water quality monitoring characterized by low-cost, high-reliability, and high-sensitivity.