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From March to December 2017, thirteensampling sites at four rivers in Jiangyin were investigated to construct phytoplanktonic index of biotic integrity (P-IBI), evaluate river health and analyze the correlation between P-IBI and environmental factors. Through the analysis of the degree of human disturbance, the reference sites and damaged sites were divided. The discriminating power analysis and Pearson correlation analysis were conducted on 18 candidate parameters. Finally, five parameters were selected, namely, the percentage of Bacillariophyta, the percentage of Chlorophyta, the percentage of Phormidium, the Shannon-Wiener index and the biomass of phytoplankton. Ratio method was used to get the score of each parameter and the total score of P was obtained after accumulating. The results show that the investigated dominant species of phytoplankton in the river are Oscillatoria, Anabaena and Euglena, eutrophication of water body is severe and water quality is generally poor. P has a high correlation with the river flow velocity, transparency, salinity and conductivity and soil erosion caused by human activities is the main influencing factor of the structure of phytoplankton population structure. P of four rivers is generally consistent with the water quality data. The better the water quality, the higher the P-IBI score. It is feasible to use P-IBI to evaluate river health. [ABSTRACT FROM AUTHOR]