Abstract: To investigate the community characteristics of phytoplankton and the influence of environmental factors on spatial-temporal distribution in Tianjin section of the Beijing-Hangzhou Grand Canal during the dry and wet seasons, comprehensive surveys were conducted during both dry (April 2020) and wet (July 2020) seasons. The study employed qualitative and quantitative analyses of environmental parameters and phytoplankton community structure, utilizing multiple assessment methods including single-factor evaluation, comprehensive trophic level index (TLI), and biodiversity index to evaluate water quality and aquatic ecological conditions. Relationships between environmental factors and phytoplankton communities were examined through Pearson correlation analysis and redundancy analysis (RDA). Results indicated that the Beijing-Hangzhou Grand Canal in Tianjin section maintained an alkaline condition with notable spatial variations in water quality. During the dry season, the water quality in the urban area was excellent, that in the Beichen section was good, and the remaining sections exhibited mild pollution. The water nutritional status was moderate throughout the section, except for the Wuqing section, which showed mild eutrophication. While in the wet season, the Wuqing section and Jinghai section presented mild pollution, the Xiqing section showed moderate pollution, and the overall water nutritional status was mildly eutrophic. At the same time, a total of 109 phytoplankton species (belonging to 56 genera, 7 phyla) were identified in the study. Among them, Chlorophyta (48 species, 44.0%), Bacillariophyta (27 species, 24.8%), and Cyanobacteria (15 species, 13.8%) constituted the dominant groups in the dry season. In the wet season, 154 phytoplankton species (belonging to 73 genera, 7 phyla) were identified, with Chlorophyta (65 species, 42.2%), Cyanobacteria (38 species, 24.7%), Bacillariophyta (24 species, 15.6%), and Euglenophyta (16 species, 10.4%) comprising the main components. The average biomass of phytoplankton in the dry season was 16.86 mg/L and in the wet season it was 786.79 mg/L. Biodiversity index evaluations showed that the Tianjin section of the Beijing-Hangzhou Grand Canal was moderately polluted, with more severe water pollution in the dry season than in the wet season. Pearson correlation analysis indicated that the Pielou index of phytoplankton was significantly negatively correlated with biochemical oxygen demand (BOD) in the dry season. In the wet season, the Shannon-Wiener index was significantly positively correlated with total nitrogen (TN), nitrate nitrogen (NO₃⁻-N) and Chlorophyll a (Chla), while the Pielou index was significantly negatively correlated with total nitrogen (TN) and nitrate nitrogen (NO₃⁻-N). Redundancy analysis (RDA) revealed that pH, dissolved oxygen (DO), temperature (T), permanganate index (COD_Mn), chemical oxygen demand (COD) and biochemical oxygen demand (BOD) primarily influenced the plankton community structure in the dry season, while in wet season, dissolved oxygen (DO), temperature (T), biochemical oxygen demand (BOD) and ammonia nitrogen (NH₄⁺-N) were the main factors affecting the phytoplankton community structure.