Abstract: To solve the water environment dispute problem of the trans-boundary section in plain river network, it is particularly important to define the pollutant sources in the trans-boundary section. Taking Jiashan County as a case, a water environment mathematical model in Jiashan County was established. By using the influence weight analysis method of the internal and external pollutant sources and the point and surface pollutant sources in the trans-boundary section, the results of calculation and analysis show that the main factor affecting the water environment of trans-boundary section is the important source of the internal area. The pollutants sources analysis of each trans-boundary section to the important point sources in the internal area was carried out by using pollutant flux index. Results show that the large discharge sewage treatment plant closed to the trans-boundary section of the cross section has a large influence on the water environment of the trans-boundary section. It is of great significance to solve the trans-boundary water environment dispute problem by rationally controlling the major sources of the greatest influence weight in trans-boundary section.