Abstract: On the basis of the theoretical solution to the concentration distribution under the conditions of the invariableness continuous plumb line source in the straight width rectangle of the river, this paper discusses the analytic method of the pollutant mixing zone in rivers, and deduces the largest length of the pollution mixing zone, the largest width and its corresponding formula of longitudinal coordinate, the area and the largest allowable pollutant load. The example analysis of the pollutant mixing zone is conducted. The result indicates that the analytic method can be used for the analytic gist of the outfall's pollutant mixing zone in the allowable range and the largest allowable pollutant load, and the theoretical calculation is faster, more flexible, more convenient and more practicable than the 2D water quality model used for the calculation of the pollutant mixing zone. The paper gives the largest length calculation formula of the non-conservative substances pollutant mixing zone and the condition, neglecting the reaction and the decay effect and dealing with it based on the conservative substance. The formula of the decay number (De) is as follows: De≤0.027.