Using on-line pre-chlorine and sulfate removal technique, a rapid and direct method was established for the simultaneous determination of nitrite-N, nitrate-N and phosphate-P in seawater by ion chromatography (IC) with suppressed conductivity detection. In this method, an IonPac AG23 column was used as the enrichment column, an IonPac AS23 column with high capacity as the analytical column, and KOH solution as the eluent for gradient elution produced by an eluent generator on-line automatically. The results showed that the determination of the targets was not interfered when a seawater sample diluted 5 to 10 times was directly injected. The method detection limits (MDL) of nitrite-N, nitrate-N and phosphate-P were 0.3, 0.4 and 0.2 microg/L respectively, under the conditions of 500 microL sample volume and 1 mL/min elution rate. The linear ranges of nitrite-N, nitrate-N and phosphate-P were 10 - 500 microg/L, 14 - 680 microg/L and 3.4 - 170 microg/L respectively, and the correlation coefficients (r) were all greater than 0. 999 0. The average recoveries of the 3 analytes spiked in a sample ranged from 92% to 106% with the relative standard deviations (RSDs, n = 6) of 1.2% - 7.7%. The whole sample analysis process can be completed in 13 min. The method has the advantage of simple operation, reagent-free and pollution-free, and can meet the quantitative analysis requirements of seawater.