Vector radiative transfer (RT) and first-order multiple scattering (FOMS) theories are often used for analyzing the depolarization effect by the random medium. However, the numerical solution for the RT theory is limited to moderate-sized particles because of the numerical stability. In order to analyze the depolarization effect by large particles near the backscattering direction, we obtained the second-order multiple scattering (SOMS) theory for the vector RT theory. The derivation was based on the second-order solution of each Fourier component of the Stokes vector. The numerical results were compared with FOMS and RT theories. It was shown that the SOMS theory was most useful for large particles and near the backscattering direction. Experimental results for large spherical particles were compared with the SOMS theory. The second-order ladder term which was included in the SOMS theory was not sufficient to explain the sharp peak observed in the backscattering direction in the depolarized intensity. The peak appears to be caused by the backscattering enhancement effect.