Abstract: The self-purification process is an important factor influencing changes in the water quality of rivers and lakes, and the degradation coefficient of pollutants is the key parameter determining the self-purification ability of water bodies.The quantitative analysis of the self-purification function of the channel in long-distance water transport is of great significance for understanding the water quality change process and distinguishing the influence of other external factors on the water quality change.Using permanganate index(CODMn) and ammonia nitrogen(NH3-N) as the research subjects, the pollution degradation coefficients of typical sections of main canal of Middle Route of South-to-North Water Diversion Project were determined using indoor experimental simulation methods, and the simulated self-purification decay process was compared with the actual water quality changes.The results showed that:(1) The pollution degradation coefficients for the main canal were relatively low compared to natural rivers.The degradation coefficients of CODMn of Taocha, Zhengwan and Daanshe were 0.055 d-1,0.049 d-1 and 0.032 d-1 respectively, while the degradation coefficients of NH3-N were 0.003 d-1,0.003 d-1 and 0.009 d-1 respectively.Considering that the actual components of the permanganate index were complex, the channel degradation coefficient may be lower than the simulation results.(2) The simulated self-purification decay process was significantly influenced by temperature and flow rate.Under different conditions, the permanganate index in the main channel decreased from 1.8 mg/L to a range of 0.228 to 1.042 mg/L,while ammonia nitrogen concentration decreased from 0.034 mg/L to a range of 0.026 to 0.030 mg/L.Due to the external inputs from algae, atmospheric deposition, and surface runoff, the permanganate index along the main channel ranged between 1.8 to 2.4 mg/L(in 2019),remaining relatively stable; while ammonia nitrogen concentrations ranged from 0.029 to 0.096 mg/L(in 2019),showing an increasing trend.Under ideal conditions, the self-purification process can reduce the average proportion of changes in permanganate index and ammonia nitrogen by 81.81% and 30.37%,respectively.The research results can provide a reference for further understanding the changes of water quality along the main canal of the Middle Route of South-to-North Water Diversion Project.