The serious non - point source pollution and cross - pollution in plain tidal river networks presented problems of environmental pollution and environmental safety. Because of deficiency in monitoring data and research, The related monitoring management is yet to be improved. Difficulties such as identifying the extent of the non - point pollution and quantitatively forecasting the pollution of regions in defect of data need to be resolved with urgency. This thesis took Nantong plain tidal river networks for example, discussed the indeterminacy of the non - point pollution and the monitoring and analyzing method, and brought forward the monitoring method towards the polder and plain areas respectively.