Abstract: In order to clarify the species composition and quantity distribution characteristics in summer in tideland of Jiangsu, scene investigation and sampling was done in August 2014. A total of 73 species from Baeillariophyta, Pyrophyta, Chrysophyta and Chlorophyta were collected, including 68 species (93.15%) of Baeillariophyta, 4 species (5.48%) of Pyrophyta, 1species (1.37%) of Chrysophyta and 1species (1.37%) of Chlorophyta, the average density were 5.65×10⁶ ind. /m³, 4.75×10⁴ ind. /m³, 92.36 ind. /m³ and 51.31 ind. /m³ respectively. The grand average density was 8.44×10⁶ ind. /m³. Baeillariophyta was the dominant group. According to the degree of dominance, Skeletonema costatum (Y=0.48), Eucampia zoodiacus(Y=0.10) and Coscinodiscus argus (Y=0.02)was the dominant species, the average density was 3.23×10⁶ ind. /m³, 1.44×10⁶ ind. /m³ and 1.28×10⁵ ind. /m³ respectively. Grand average phytoplankton diversity index of the tideland was 1.48. The quantitative distribution characteristics of phytoplankton showed that species number and density was higher in seacoast and south, lower in offshore and north. Phytoplankton diversity in radial submarine sand ridges was significantly higher northern bank of Yangtze estuary, abandoned Yellow River delta and Haizhou bay. As the increasing of the nutriment discharge, species number and biodiversity of the phytoplankton community were decreased, dominant species were simplified, and red tide organism such as Skeletonema costatum and Eucampia zoodiacus became the main dominant species. Quantity distribution of the Skeletonema costatum and Eucampia zoodiacus showed that northern bank of Yangtze estuary, southern radial submarine sand ridges, Sheyang estuary, Zhongshan estuary and Guanhe estuary were in high HABs (harmful algal blooms) risk area. HABs risk distribution showed that seacoast and south area were at high risk.