Abstract: Rare earth elements are not only important strategic resources, but also an indicator of the geochemical evolution process. The content of rare earth elements in seawater is very low. Due to the influence of high salt matrix, detecting the accurate concentration is s always difficulty. The purpose of this study is to remove the background interference of seawater matrix through an online desalination device, enrich rare earth elements and quantitatively detect 14 rare earth elements with inductively coupled plasma mass spectrometry (ICP-MS) technology. Tested with blank samples, it was found that the quantification limit for each rare earth element detection was between 0.01 and 0.50 ng/L. The linear evaluation of the standard curve shows that the linear coefficient r² of each element's standard curve is greater than 0.999. We analyzed the impact of different pH values on the detection of samples and found that the accuracy was better between pH 2 and 6. The parallelism and accuracy of the detection were evaluated by detection of NASS-6 standard seawater samples and comparison with the standard values. The STD values of the three tests were between 0.01 and 0.16, and the results were close to the perious researches. Experiment showed that the recovery rates of each element ranged from 94.81% to 105.82% demonstrating a good detection accuracy. Detected pore water samples and plotted rare earth element distribution curves verify that the curve smoothness was good proving that this method can be used for the real samples. In summary, this study can provide technical support the rapid online detection for rare earth elements in seawater. The content of rare earth elements in seawater can be used not only to indicate marine geochemical activities but also to trace pollutants. The discussion in this paper can providea technical support for further researchs.