Abstract: A large eddy simulation approach for the water quality based upon the weakly compressible flow is developed,and the relationship between the diffusion coefficients and the average velocity is established. The finite volume approach in space and predictor-corrector method in time is used,and "partial slip condition" is applied to the solid wall boundary condition. This model is employed in simulating the concentration distribution of the pollutant transport at the cross area between the Jialing River and the Yangtze River,and the results show that the transverse diffusion is weak and the bank-side pollutant zone appears when the discharge of the branch river is relatively small,on the contrary,the transverse diffusion is strong and the pollutant zone moves to the heart of the main river when the discharge of the branch river is relatively large.