Abstract: To construct a phytoplankton biotic integrity index system suitable for the Tuojiang River basin and evaluate the aquatic ecological health, a survey of the current status of phytoplankton in the Tuojiang River basin was conducted during the low-flow period (February) and high-flow period (August) of 2023. The survey results indicated that a total of 101 genera from 9 phyla of phytoplankton were identified, with the community compositions during the high-flow and low-flow periods being green algae + diatoms + cyanobacteria type and diatoms + green algae + cryptophytes + dinoflagellates type, respectively. Based on the Shannon diversity index (H) water quality classification standards and the analysis of human disturbance levels to determine the reference point, Pearson correlation analysis was conducted on 25 widely used and highly selected candidate parameters, ultimately 5 core parameters for constructing the P-IBI system in the Tuojiang River basin were selected: M8 percentage of green algae cell density, M13 percentage of cyanobacteria individual density, M18 cyanobacteria biomass, M21 percentage of biomass of the top three dominant species, and M22 Shannon diversity index. A scoring method was used to uniformly score the 5 core parameters at each sampling site, and a scoring interval was given to divide the water quality health standard levels. The evaluation results showed that the ecological conditions at the upstream Mian River sites TJ1 and TJ3 were the best, with the rest mostly being "sub-health" and "average," and no "poor" or "extremely poor" sites. It can be seen that the overall phytoplankton biotic integrity in the Tuojiang River basin is relatively good. Compared to the severe water pollution two decades ago, the water quality health of the Tuojiang River has significantly improved through a series of protection and management measures in recent years.