Abstract: After impoundment of the Three Gorges reservoir， there is a great variation in the water quality in two rivers (the Yangtze River and the Jialing River) at the confluence nearby the main urban areas of Chongqing city. In order to study the change law of the pollution mixing zone， a two-dimensional water quality model is used. Based on the generalized numerical area， the pollution mixing zone is simulated numerically and the boundary features are analyzed. Then a comprehensive analysis considering different guarantee rates， different sewage loads and different outfalls is made in order to reveal the characteristic of the pollution mixing zone. The research results show that the larger the reach flow， the longer the pollution mixing zone is， so the straight reach is relatively long and the curved reach is relatively wide in the pollution mixing zone. And the scope of the zone is proportional to the sewage load.