A time-varying composite scattering of a moving ship on a two-dimensional non-linear sea surface is modelled with the help of the Doppler spectrum in high-frequency bands. A non-linear sea surface is simulated to approximate a real sea environment with high-order terms of the solution to hydrodynamic equations. Numerical simulations are based on the principle of a quasi-stationary algorithm (QSA), which solves time domain problems with frequency domain methods. A four-path model is modified to calculate the composite scattering field, which is utilized for the analysis of the Doppler spectrum. With depth-buffer technology, self-shadowing of the complex physical model is eliminated. Correspondence of the numerical results with experimental data proves that the inclusion of the non-linear interaction term and wind-speed influence is reasonable. The simulated Doppler spectrum of the composite scattering shows a functional dependence of time-evolved scattering components on incidence direction and sea state.