The spatio-temporal distribution of phytoplankton is influenced by physical and chemical factors such as water temperature, salinity, dissolved oxygen, pH, and nutrients. Nitrogen (N) and phosphoros (P) are essential for the growth of phytoplankton which play a key role in determining the abundance of phytoplankton. The species composition, community similarity, and diversity of phytoplankton affect the structure and function of ocean ecosystems. Marine phytoplankton can be used as a rapid and sensitive indicator of environmental changes. The coastal region of Qinhuangdao is used by fish, shrimp, and crabs for spawning and feeding. However, this region has experienced an increase in the frequency of red tides as a result of pollution and marine development.We evaluated the species composition, spatio-temporal distribution, community structure of phytoplankton and the relationship between phytoplankton abundance and environmental factors based on data collected during five surveys conducted from May to September, 2010. We identified 96 species belonging to 46 genera of phytoplankton. The phytoplankton community was primarily composed of Bacillariophyta and Pyrrophyta. The majority were neritic species or cosmopolitan species which were eurythermal and euryhaline, while the remainder was oceanic warm water or oceanic species. The dominant species include Chaetoceros A total of 48 red tide causative species were observed (~50% of total species). The mean phytoplankton abundance ranged from 6.20×10⁴ cells/m³ to 4129.53×10⁴ cells/m³. Abundance peaked in September 2010 and was lowest in July 2010. The and community structure was healthy. The highest indices of community structure and biodiversity were observed in September, and biodiversity was lowest in June. The community similarity of phytoplankton was relatively high and the range in similarity was from 40% to 70% from May to September. The community similarity was ranked in order: September>July>August>May>June. The average abundance of phytoplankton was significantly negatively correlated with salinity(=37) and significantly positively correlated with inorganic nitrogen nutrients (=37). We conclude that inorganic nitrogen is the primary factor explaining the increase in phytoplankton abundance during the study period. A canonical correspondence analysis of the relationship between phytoplankton species and environmental factors indicated that N/P, salinity, nitrate, silicate, nitrite, and water temperature were the main environmental factors which influenced the community structure of phytoplankton in the coastal areas of Qinhuangdao.