Offshore wind turbines are mainly built in shallow water, so the nonlinear characteristics of the wave forces are significant. The high order nonlinear wave velocity fields and acceleration fields were studied on the basis of the stream function wave theory. The nonlinear wave forces of offshore wind turbines were calculated with the Morison equation and the Stream Function Wave Theory. The free surface influences on the nonlinear wave forces of offshore wind turbines were studied. The characteristics of the nonlinear wave forces of piles of different diameters in shallow water were also investigated. To verify the correctness of the calculation results and compare the characteristics of the nonlinear wave force, the calculation results were compared with the linear wave calculation results. The results show that the maximum nonlinear wave force is 26.6% larger than the maximum linear wave force, and the free surface influences generate 14.7% relative errors. In offshore wind turbine design, the application of the nonlinear wave theory will lead to more accurate wave forces on offshore wind turbines.