Abstract: Herein, through optimization of measurement conditions and standard verification, a method for the analyzing 14 rare earth elements in sea water with inductively coupled plasma mass spectrometry(ICP-MS)was developed, by adding NH₃·H₂O directly into the seawater to form Mg(OH)₂ co-precipitation.After the co-precipitation and centrifugation operation, the main matrix materials were separated, and the enrichment of the target rare earth elements was achieved.The optimum conditions for the co-precipitation of magnesium hydroxide were as follows:0.35 mL of 1:1 NH₃·H₂O solution added in seawater as precipitation matrix, cleaning the precipitation one time for matrix separation and 5 min of the deposition time.The recoveries of the method were in the range of 88.7%~107.1%; the method blanks of 14 kinds of rare earth elements covered 0.008×10^-12~0.441×10^-12, the method quantitative limits were of 0.054×10^-12~0.423×10^-12 and the relative standard deviations (RSDs) 2.8%~8.6%.The result of NASS-6 analysis by applying this method was in good agreement with the literature report.The method is of high accuracy, good precision and simple operation, which well meets the requirement for determination of rare earth elements in seawater in large quantities.