By using iterative method to solve the vector radiative transfer equation of discrete scatterers with randomly rough under-boundary, the back-scattering coefficient is derived, and is applied to the two-scale model of sea surface with foam scatterers driven by strong wind.By employing the modified probability density function of Cox and Munk's, and Pierson's sea spectrum, numerical results of polarized back-scattering are calculated.The functional dependence on wind speed and direction, observation angle, polarization and other parameters are discussed, and theoretical results are favorably matched with experimental data.