Abstract: According to phytoplankton investigation data in spring and summer in Fujian sea area during 2003 to 2004, this paper analyzed and studied the species composition and quantity distribution characteristics in the area. The results showed that 111 species of phytoplankton were identified with 81 species of diatoms occupying 73.0%; the total biomass of phytoplankton in summer was more than that in spring with average 30.9104 indm-3 and 2 645.6104 indm-3, respectively. The total biomass of Mindong fishing ground in spring was highest, Minzhong fishing ground was lowest; but it was contrarily in summer. The total biomass in Minzhong and Minnan-Taiwan bank fishing ground obviously increased with boosting up by warm current, and Minnan-Taiwan bank fishing ground was highest and Minzhong fishing ground was followed with the obvious density area，the biomass was over 40 000104 indm-3. The dominant species were tropical near-oceanic group and tropical oceanic high-salinity group as Chaetoceros lorenzianus, Stephanopyxis palmerana, Rhizosolenia stolterfothii, Trichodesmium thiebautii, Trichodesmium hildebrandtii and Dactylioselen mediterraneus. The species diversity index was low, but the quantity of dominant species was high. The dense areas of phytoplankton were the main fishing ground of trawler and light seine. Compared with the data of historical and adjacent waters, the fluctuation of majority index was large in years; the biomass of phytoplankton increased obviously but the dense area was changed slighly.