During spring and autumn of 2006, the investigations on abundance, carbon biomass and distribution of picoplankton were carried out in the southern Huanghai Sea (Yellow Sea, sHS). Three groups of picoplankton-Synechococcus (Syn), Picoeukaryotes (Peuk) and heterotrophic bacteria (BAG) were identified, but Prochlorococcus (Pro) was undetected. The average abundance of Syn and Peuk was lower in spring (5.0 and 1.3×10~3 cells/cm~3, respectively) than in autumn (92.4 and 2.7×10~3 cells/cm~3, respectively), but it was opposite for BAC (1.3 and 0.7×10~6 cells/cm~3 in spring and autumn, respectively). And the total carbon biomass of picoplankton was higher in spring (37.23±11.67) rag/m~3 than in autumn (21.29±13.75) mg/m~3. The ratios of the three cell abundance were 5:1:1 341 and 30:1:124 in spring and autumn, respectively. And the ratios of carbon biomass of them were 5:7:362 and 9:4:4 in spring and autumn, respectively. Seasonal distribution characteristics of Syn, Peuk, BAC were quite different from each other. In spring, Syn abundance decreased in turn in the central waters (where phytoplankton bloom in spring occurred), the southern waters and inshore waters of the Shandong Peninsula (where even Syn was undetected); the high values of Peuk abundance appeared in the central and southern waters and the inshore of the Shandong Peninsula; the abundance of BAC was nearly three order of magnitude higher than that of photosynthetic picoplankton, and high values appeared in the central waters. In autumn, Syn abundance in central waters was higher than that in surrounding waters, while for Peuk abundance, it decreased in turn in the inshore waters of the Shandong Peninsula, the southern waters and the central waters; BAC presented a complicated blocky type distribution. Sub-surface maximum of each group of picopalnkton appeared in both spring and autumn. Compared with the available literatures concerning the studied area, the range of Syn abundance was larger, and the abundance of BAC was higher. In addition, the conversion factors for calculating picoplanktonic carbon biomass were discussed, with the conversion factors which are different from previous studies in the same surveyed waters. The result of regression analysis showed that there was distinct positive correlation between BAC and photosynthetic picoplankton in spring (r=0.61, P 锛*0.001), but no correlation was found in autumn.