To understand the dynamic patterns of the permanganate index (COD_Mn) and impact factors in the surface water of the mainstream of Xijiang River, seasonal investigations were conducted between Dawan of Hongshui River and Zhaoqing of Xijiang River from September 2017 to June 2018. The water quality was analyzed and evaluated basing on the present study. During the investigation, the COD_Mn concentration varied in the range of 0.73~4.83 mg/L, and the mean value was 2.67 mg/L. The seasonal COD_Mn mean value were 3.08 mg/L (June), 3.03 mg/L (December), 2.94 mg/L (September), and 1.62 mg/L (March). The 13 sample stations were clustered in three groups, and the highest content of COD_Mn was found in cluster Ш, followed by cluster I, with the lowest in cluster II. Results of pollution conditions based on COD_Mn were in the range of 0.18~1.21, whereas the corresponding water quality levels were "no pollution" to "light pollution." The temporal water pollution for the annual average content of COD_Mn in water from the 13 sites in descending order was December, June, September, and March. The water quality identification index and levels based on COD_Mn showed that at most times in one year the water quality classification belonged to class I or II, and the last times belonged to class III. Specifically, water quality in March was best with assigned to class I, followed by periods in September and December, which was belonged to class II, and the worst water quality was in June, which belonged to class II to III. Correlation analysis with other physical and chemical factors showed that COD_Mn had a significant positive correlation with water temperature and total phos phorus, and it was significantly negatively correlated with salinity, dissolved oxygen (DO), secchi depth, total phosphorus, nitrate nitrogen, nitrite nitrogen chlorophyll a, and N:P. Stepwise regression analysis showed that COD_Mn concentration was significantly affected by total phosphorus, phosphate, and DO. According to the present study, it was determined that the water environment in mainstream of Xijiang River was lightly polluted and the water quality was better than most of the other rivers in our country. It was also concluded that the hydrodynamic situation of the river and human settlements near the sampling area were the factors affecting the COD_Mn concentration. Additionally, it was determined that the reducibility of organic matter came from "point source pollution" in the investigation area, which was in common with the sources of water total phosphorus mass.