The seasonal variation and spatial distribution of silicate and Chl. a during May to October in 2009 were analyzed by taking Nandaihe coastal waters as a case study, and the relationship between them and the picophytoplankton algal bloom was also discussed. Results showed that the average concentration was 0.67±0.31 mg·L^-1 for silicate, and 2.06±1.16 ug·L^-1 for Chl. a during the research. The spatial distribution of silicate and Chl. a both decreased from inshore to offshore area. The average concentration of silicate was the highest in September, and the lowest in May, while that of Chl. a was the highest during August to September, and the lowest during May to June. During the period of picophytoplankton algal bloom, the average concentrations of silicate and Chl. a were significantly higher than those in the same period of 2008 when the picophytoplankton algal bloom didn't occur. It showed positive correlations between distribution features of silicate and Chl. a, which was more obvious in the course of picophytoplankton algal bloom. It was analyzed that the mass propagation of picophytoplankton inhibited the normal reproduction of the phytoplankton such as diatom which utilized silicate for growth.