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 studied.The finite element method is used for conducting numerical analysis and the stability of the numerical solution is also investigated.Comparing the calculation data with the measured one, it is shown that the obtained result has better accuracy.This paper treats seaward firth as a dynamic system, divide the solution domain into finite elements and find the solution by a punitive function,variable size combinatorial method and non-linear optimization solution.In order to ensure solution stability,use non-dimension variables and take time-step as control condition.