Abstract: The two scale method (TSM) is widely used for electromagnetic scattering simulation of randomly rough sea surfaces. The geometric roughness spectrum and truncation wavenumber are two key factors directly affecting the accuracy of TSM simulation. Taking the geophysical model function (GMF) developed with the observation data as a reference, these two key factors are quantitatively studied in this paper. The empirical function of the truncation wavenumber model is fitted with the regression methods. The simulation results show that the Apel spectrum is more suitable for simulating the backscattering of rough sea surface in C, X, and Ku bands, and the simulation error with the proposed empirical function of truncation wavenumber under different conditions are all less than 1 dB. Comparative analysis with existing truncated wavenumber models shows that the proposed truncation wavenumber model can effectively calculate the Bragg scattering contribution from the roughness spectrum, and the model function has a wider applicable band and observation angle.