Abstract: On the basis of analyzing the flow regime and pollution pattern of the sea area of Jiaolu Jiang Firth,the pollutant nonlinear diffusion phenomenum in the seaward firth with mixed-up salt and fresh water is taken into consideration.The finite element method is used for conducting numerical analysis and the stability of the numerical solution is investigated.Comparing the calculated value with the measured one,it is verified that the result obtained has better accuracy.