Abstract: In order to analyze the distribution characteristics and influencing factors of phytoplankton functional groups in the Haihe River Basin, surveys were conducted at 15 sampling sites in the Tianjin section of the Haihe River Basin in the dry season (April 2023) and the rainy season (September 2023). This study aimed to explore the species composition and functional group distribution characteristics of phytoplankton and the correlation between environmental factors and phytoplankton functional groups in different hydrological periods. A total of 140 of phytoplankton species from 7 phyla were identified, the species numbers were Bacillariophyta, Cyanophyta and Euglenophyta. The dominant species (dominance≥0.02) in the study area included Synedra acus, Aphanizomenon sp., Gymnodinium sp., Cryptomonas ovata, Melosira granulate and Euglena sp.. In addition, 23 functional groups were identified, among which A, D, H1, L_O, MP, P, TB and Y were the major functional groups. The results of RDA analyses showed that water temperature (WT) and permanganate index (COD_Mn) were important factors affecting phytoplankton functional groups during the wet season, while during the dry season, the key environmental influence was the permanganate index (COD_Mn). Furthermore, the water quality evaluation using the Shannon-Wiener diversity index and the Ecological index evaluation method based on functional group (Q_r index) indicated that the water quality in the investigated area was clean, providing consistent and positive results. The study showed that functional groups can directly reflect the phytoplankton characteristics in the Tianjin section of the Haihe River Basin, and Q_r index can objectively reflect the water quality in the Tianjin section of the Haihe River Basin.