The Mao River Basin in the Pengshan District of Meishan， Sichuan Province was used as an example to analyse pollution traceability comprehensively and accurately. Based on the consideration of changes in hydrological conditions and the spatial distribution of pollution sources， a mathematical model of the basin water environment was established to analyse the weight of pollution sources during different periods by pollution contribution fluxes and equivalent pollution load ratios. The results indicate that：（1） Pollution from urban domestic sewage in the Mao River basin is the main contributor to chemical oxygen demand（COD） and ammonia nitrogen（NH₄⁺-N） river inflow， accounting for 52.14% and 50.74%， respectively. This pollution is mainly located in the old urban area of Pengshan. Farming is the main contributor to the amount of total phosphorus（TP） in the river， accounting for 51.32%， and is distributed in Tongji weir irrigation district and other pollution source areas. Industrial pollution sources contributed the least to the amount of COD， NH₄⁺-N and TP in the river， accounting for no more than 2%. The simulation results of a one-dimensional non-stationary model in the Mao River Basin showed that the percent biases（PBIAS） of COD， NH₄⁺-N and TP in each period were less than or equal to 25%， which was more consistent with the actual values.（3） During the overflow influence period， the pollution sources in Pengshan urban area have the greatest impact on the water quality of the Qiaojiangqiao section. The weights of water quality influence are 67.2% for COD， 67.3% for NH₄⁺-N， and 51.7% for TP， respectively. The pollution characteristics during the rainfall period and overflow influence period are similar. However， the dilution effect of heavy rainfall makes the water quality avoid exceeding the standard. During the irrigation recession period， the primary source of pollution is agricultural. The Tongji weir irrigation area contributes the most to the total pollution（37.6%） at the Qiaojiangqiao section.