In order to assess the impact of the Fukushima derived Pu isotopes on seawater, a new analytical method to rapidly determine Pu isotopes in seawater by SF-ICP-MS including Fe(OH)2 primary co-precipitation, CaF2/LaF3 secondary co-precipitation and TEVA+UTEVA+DGA extraction chromatographic separation was established. High concentration efficiency (∼100%) and high U decontamination factor (∼107) were achieved. The plutonium chemical recoveries were 74-88% with the mean of 83 ± 5%. The precisions for both 240Pu/239Pu atom ratios and 239+240Pu activity concentrations were less than 5% when 15 L of seawater samples with the typical 239+240Pu activity of the Northwest Pacific were measured. It just needs 12 hours to determine plutonium using this new method. The limit of detection (LOD) for 239Pu and 240Pu were both 0.08 fg/mL, corresponding to 0.01 mBq/m3 for 239Pu and 0.05 mBq/m3 for 240Pu when a 15 L volume of seawater was measured. This method was applied to determine the seawater samples collected 446-1316 km off the FDNPP accident site in the Northwest Pacific in July of 2013. The obtained 239+240Pu activity concentrations of 1.21-2.19 mBq/m3 and the 240Pu/239Pu atom ratios of 0.198-0.322 suggested that there was no significant Pu contamination from the accident to the Northwest Pacific.