Abstract: Short sequence of hydrological data of small and medium rivers is unable to delimit pollution load red line scientifically, reasonably by deterministic method in calculating water environmental capacity. Considering the uncertainty of river water environment system parameters, the extended blind number method and Monte Carlo method are used to calculate the water environment capacity, and the staged limiting pollution load red line is delineated by combining the target of water function area and the discharge scale of pollutants. Taking Hexi River in Danyang City as an example, under the present condition of pollution source discharge, the amount of ammonia nitrogen entering the river is required to reduce by 0.06t from March to June, the limiting pollution load red line of ammonia nitrogen was 13.67 kg/d in the rest of the year. The example proves that the two uncertainty methods are feasible and effective in the calculation of water environmental capacity of small and medium rivers. It is considered that the calculation results of uncertainty methods are more scientific and reasonable in the definition of staged limiting pollution load red line.