An analytical method of on-line injection technique including on-line dilution and on-line preconcentration with inductively coupled plasma mass spectrometry ( ICP-MS ) was established for the determination of trace metals in seawater, such as Cr, Mo, Cu, Pb, Zn, Cd, Fe, Mn, Ni, V, Co. This method could automatically make standard curves and match matrix, and by the on-line dilution and preconcentration modes, 11 trace metals could be analyzed directly. With high-level automation, CASS-5 and NASS-6 seawater standard samples were analyzed accurately by this method. The recovery of metals in seawater sample addition was between 80% and 115%. The method quantitation limits (μg/L ) of theses metals in seawater were 0. 06(Cr), 0. 06(Mo), 0. 01(Cu), 0. 005(Pb), 0. 01(Zn), 0. 006(Cd), 0. 03 (Fe), 0. 02(Mn), 0. 01(Ni), 0. 01(V), and 0. 01 (Co), respectively. This method was applied to analyze seawater samples of various salinities in Zhejiang coast, and the analysis results were consistent with results of conventional atomic absorption method.