In order to controll pollutant discharge and identifying the red line of restricting pollutants in the water function area, taking the four water function areas in the Yishusi River basin in Shandong Province as an example, the degradation coefficients and variations of water body COD and ammonia nitrogen were analyzed.. The empirical formula for degradation coefficient of water pollutant, related to the velocity, were fitted. The pollutant carrying capacity of COD and NH3—N of four water function areas in 2015 were calculated by using one-dimensional water quality model, and it was verified that pollutant carrying capacities of COD and NH3—N at the hydrological guaranteed rates of 75% and 90% were defined as the capacity limit red lines in 2020 and 2025 for the studied region, respectively. Finally, the forecast was conducted. The results showed that, the permissible pollution bearing capacity of COD and NH3—N in 2015, 2020 and 2025 were 2264.03 and 34.48，1566.4 and 21.95, and 887.71 and 15.16 tons, respectively.The pollutant reduction rates of COD and NH3—N were 30.81% and 36.34% in 2020, and 60.79% and 56.03% in 2025, respectively.