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 phosphorus(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 reflecting environmental changes.The offshore area of Qinhuangdao is a spawning and feeding ground for fish,shrimp and crabs.However,this region has experienced an increase in frequency of red tides as a result of pollution and marine development.In 2010,red tides began in late May and ended in August.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 from May to September in 2010.The phytoplankton,belonging to 96 species,46 genera,were identified.The phytoplankton community was primarily composed of Bacillariophyta and Pyrrophyta.The majority were neritic species or eurythermal and euryhaline cosmopolitan species,while the remainders were oceanic warm species or pelagic species.The dominant species included Noctiluca scintillans,Rhizosolenia stolterfothii,Chaetoceros spp.,Coscinodiscus spp.,and Bacteriastrum spp..A total of 48 red tide causative species were observed,accounting for 50%of total phytoplankton species.The average phytoplankton abundance ranged from 6.20 × 10 cells/m~3 to 4 129.53 × 10 cells/m~3.Abundance peaked in September2010 and reached the lowest value in July 2010.The diversity index values were all greater than 1.0,suggested that biodiversity and community structure was healthy.The highest indices of community structure and biodiversity were observed in September,and biodiversity was the lowest in June.The community similarity of phytoplankton was relatively high and the similarity range was 40%-70%from May to September.The community similarity of phytoplankton successively were September > July > August > May > June.The average abundance of phytoplankton had significant negative correlation with salinity(r =-0.734,P < 0.01,N=37) and significant positive correlation with inorganic nitrogen nutrients(r =0.753,P <0.01,N = 37).We concluded that inorganic nitrogen was the primary factor explaining the increase in phytoplankton abundance during the survey 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 influencing community structure of phytoplankton in Qinhuangdao waters. 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 phosphorus (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 reflecting environmental changes. The offshore area of ​​Qinhuangdao is a spawning and feeding ground for fish, shrimp and crabs.However, this region has experienced an increase in frequency of red tides as a result of pollution and marine development. 2010, red tides began in late May and ended in August.We think the species composition , spatio-temporal distribution, community structure of phytoplankton and the relationship between phytoplankton abundance and environmental factors based o n data collected from May to September in 2010. The phytoplankton, belonging to 96 species, 46 genera, were identified. The phytoplankton community was primarily composed of Bacillariophyta and Pyrrophyta. the majority were neritic species or eurythermal and euryhaline cosmopolitan species, while the remainders were oceanic warm species or pelagic species. The dominant species included Noctiluca scintillans, Rhizosolenia stolterfothii, Chaetoceros spp., Coscinodiscus spp., and Bacteriastrum spp. A total of 48 red tide causative species were observed, accounting for 50% of total phytoplankton species The average phytoplankton abundance ranged from 6.20 × 10 cells / m 3 to 4 129.53 × 10 cells / m 3. Abundance peaked in September 2010 and reached the lowest value in July 2010.The diversity index values ​​were all greater than 1.0, suggested that biodiversity and community structure was healthy.The highest indices of community structure and biodiversity were observed in September, and biodiversity was the lowes tin June. The community similarity of phytoplankton was relatively high and the similarity range was 40% -70% from May to September. The community similarity of phytoplankton were were> July> August> May> June. The average abundance of phytoplankton had significant negative correlation with salinity (r = -0.734, P <0.01, N = 37) and significant positive correlation with inorganic nitrogen nutrients (r = 0.753, P <0.01, N = 37) .We concluded that inorganic nitrogen was the primary factor explaining the increase in phytoplankton abundance during the survey 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 influencing community structure of phytoplankton in Qinhuangdao waters.