The phytoplankton community in the ecological interception ditch of the farm demonstration base approach to the eastern Lake Chaohu were analyzed in a six months-experiments. About 75 species of phytoplankton were identified. The ecological interception ditch reduced the runoff of nitrogen and phosphorus significantly, but it didn't change the phytoplankton community. Chlorophytes, Cyanophytes and Bacillariophytes were dominant in the ecological interception ditch. The proportion of Cyanophytes in phytoplankton community increased in the outlet contrast with the inlet. Microcystis aeruginosa, Chlamydomonas microsphaera and Cyclotell meneghiniana were main species in the ecological interception ditch. There are significant relationships between the biomass of phytoplankton and lg TDP, lg TP and TN/TP. The Simpson's diversity index of phytoplankton suggested the water quality of the ecological interception ditch was clean, only little polluted in July. Canonical corespondence analysis(CCA) exhibited the distribution of phytoplankton was mainly affected by TN/TP. Our results suggested macrophytes in ecological interception ditch reduce the runoff of nitrogen and phosphorus, while didn't prevent the dominance of Cyanobacteria. The P-limitation of phytoplankton was the main factor affecting the growth of algae, a reduction of phosphorus would result in the control of algae and the cleanup of water quality.