The phytoplankton species and their community characteristics in May and August of 2014 were studied at the 9 sites in the Haihe River mouth. The results show that 48 species of phytoplankton were found in the Haihe River mouth, belonging to 33 genera, 21 families, 7 genera. The dominant species are diatoms, cyanobacteria and green algae, which is accounts for 75.83%. Microcystis ichthyoblabe Kutz and skeletonema costatum are main species. The average density of phytoplankton is 1654.21×104 cells/L. The density is highest at Site No.3 in August. The average of biomass is 1.235 mg/L in May and 1.262mg/L in August. It is slightly higher in August. The average of Shannon-Wiener diversity index (H′) is 0.97. The average of Margalef index (D) is 2.24. The average of Pielou index (J) is 0.25. The result indicates that the water body of the Haihe River mouth is medium pollution and mesotrophice level.