Aiming at the problem that the confined water usually fluctuates in coastal area and it will cause inrush accidents of foundation pit. To deal with the stability in this condition, the seepage consolidation theory was used. Then the semi-analytical solution of excess pore water pressure considering soil nonlinearity was deduced. The solution was compared with the results by finite difference method in order to verify the validity of the solution. The effects of soil nonlinearity and dynamic confined water parameters on the excess pore pressure in the aquifer and the exit gradient at the excavation surface were analyzed. The calculating results show that the excess pore pressure considering the nonlinearity of the soil is larger than that under the linear assumption at all depths. The maximum exit gradient at the excavation surface of the foundation pit increases with the increasing of the dimensionless factor. The exit gradient will be smaller without considering the nonlinearity of the soil. The analysis of stability or precipitation design with solutions under linear assumption will lead to great potential safety hazards. The nonlinear influence should be fully considered especially when the dynamic pressure water has a long cycle of change, the average water level is high, or the amplitude of the fluctuation is large.