Abstract: The permanganate index is an important indicator in the water quality management of the main canal of the South-to-North Water Diversion Middle Route Project.Based on the long-term monitoring data from 30 monitoring sections along the main canal from 2017 to 2023, the impacts of the incoming water from Taocha and the downstream proliferation of algae on the permanganate index of the canal were analyzed.The results show that the permanganate index in the main canal varies within the range of 1.4~3.5 mg/L, with significant annual fluctuations.The permanganate index of the incoming water from Taocha fluctuates between 1.4~2.4 mg/L, and is significantly correlated with the permanganate index in the main canal (standardized multiple regression coefficient is 0.902, P < 0.001).The water flow in Taocha fluctuates between 70~400 m³/s, in which some high flow brings along an increase of the permanganate index in the incoming water, however it has no significant correlation with the permanganate index of the main canal.The average algae density in the canal shows significant seasonal variations, reaching a peak from July to September each year, and it is significantly correlated with the permanganate index of the main canal (standardized multiple regression coefficient is 0.168, P < 0.01).The background concentration of the incoming water from Taocha is the most important factor affecting the permanganate index of the main canal, and the proliferation of algae in the canal leads to a certain increase in the permanganate index along the canal.When the permanganate index of the incoming water from Taocha is higher than 1.8 mg/L, the number of sections in the main canal where the permanganate index exceeds 2 mg/L is likely to reach more than a half.The research results can provide a basis for formulating water quality management measures for the main canal.