[Objective]To investigate the content of trihalomethanes (THMs) in treated water after different water treatment processes and their correlations with premanganate index, so as to provide data support for the renovation of water production process and optimization of water quality improvement.[Methods]From 2022 to 2023, seven centralized water supply units using raw water from Qingcaosha reservoir were selected as the testing sites, among which three units with the conventional treatment process, two units with the advanced treatment process, and two units with the advanced treatment process combined CO₂ treatment. Monthly water quality testing data were collected, focusing on testing the concentration variations of THMs, trichloromethane, dibromochloromethane, bromodichloromethane, bromoform, and permanganate index.[Results]The comparison between conventional treatment process and advanced treatment process demonstrated that the conventional treatment process exhibited significantly higher concentrations of trihalomethanes, trichloromethane, bromodichloromethane, and permanganate index in water samples (all P<0.05). When comparing conventional treatment process with advanced treatment process combined with carbon dioxide treatment, the conventional treatment process showed significantly elevated levels of trihalomethanes, dibromochloromethane, bromodichloromethane, and permanganate index (all P<0.05). No statistically significant differences were observed in the comparison of various indicators between advanced treatment process and advanced treatment process combined with carbon dioxide treatment for any of the measured parameters (all P>0.05). Analysis of seasonal variations revealed that finished water during the high-temperature period (May to November) contained significantly higher concentrations of trihalomethanes, trichloromethane, bromodichloromethane, and tribromomethane compared to the low-temperature period (December to April of the following year) (all P<0.05). Significant positive correlations were identified between permanganate index and trihalomethanes (r=0.213, P=0.007), permanganate index and dibromochloromethane (r=0.186, P=0.019), permanganate index and bromodichloromethane (r=0.243, P=0.002), permanganate index and tribromomethane (r=0.193, P=0.014).[Conclusion]Compared to the conventional water treatment process, advanced treatment process and advanced treatment combined with CO₂ injection process can significantly reduce the concentrations of THMs in the treated effluent water. Besides, the generation of THMs is affected by seasonal temperatures, with higher concentrations of THMs, trichloromethane, bromodichloromethane, and bromoform being observed in the high-temperature season. Additionally, the permanganate index shows a significant positive correlation with THMs concentrations, indicating that the content of organic matter in the source of raw water contributes to the generation of THMs in the treated water.