Based on a bloom of Akashiwo sanguinea Hirasaka in Sishili Bay, Yantai in 2007, the relationship between the harmful algal bloom and environment parameters is discussed in this paper. During the red tide, surface seawater (0-0.5m deep) samples and bottom seawater (0.5 m to the sediment) samples were collected. Water samples for the phytoplankton study were treated with 4% neutral buffered formalin to fix the phytoplankton, and then concentrated, enumerated and identified with an inverted microscope (Olympus BX51, Japan) at 100-400 magnification. The temperature was measured in situ by a temperature meter (SWL1-1). The salinity of the water samples was determined with a salinity meter (SYC2-2). DO was analyzed by the iodometric method. COD was determined by the KMnO₄ method. Nitrate nitrogen, ammonium nitrogen and nitrite nitrogen were measured using the cadmium-copper reduction method, the indophenol blue methods, and the Griess-Ilosvay method, respectively. DIP and DRSi was measured with the phosphomolybdenum and silicomolybdenum blue method, respectively. Chl-a was determined by UV-VIS spectrophotometry (UV-2102). Dissolved inorganic nitrogen (DIN) was calculated as the sum of ammonium nitrogen, nitrite nitrogen and nitrate nitrogen. One-way ANOVA analysis, Duncan's comparison and Pearson correlation analysis were completed using by SPSS 17. The results showed that because of the rainfall, the salinity declined, while the nutrients, especially inorganic phosphate, increased, and therefore the red tide of A. sanguinea occurred. Before the red tide, the dominant species was Pseudo-nitzschia pungens Grunow, with a dominance index of 0.47 (0.42-0.52) and a diversity index of 2.63 (2.43-2.89). During the red tide, the density of A. sanguinea ranged from 1.05×10⁵ to 4.10×10⁶ cells/L; its dominance index was 0.92 (0.83-0.99),and its diversity index was 0.27 (0.15-0.64). After the red tide, Skeletonema costatum Cleve was the dominant species; its dominance index was 0.49 (0.43-0.55) and its diversity index was 2.46 (2.19-2.84). The occurrence of the red tide was positively correlated with COD, inorganic nitrogen, inorganic phosphate, eutrophication index (E) (P<0.05).but it was negatively correlated with salinity (P<0.01). Before and after the red tide, the seawater was oligotrophic and phosphorus-limited, and the chl-a concentration was low; but during the red tide, the seawater was eutrophic and phosphorus-limited, and chl-a concentration was high. Based on the assessment of the impact, inorganic phosphate, COD and salinity were the major inducing factors. Diatoms replaced dinoflagellates when the concentration of inorganic phosphate was less than 0.3 μmol/L.