The surface-directed spinodal decomposition (SDSD) in binary polymer mixture with different quench depths is investigated by numerical simulations, combining Cahn-Hilliard-Cook (CHC) theory with Flory-Huggins-de Gennes theory. The formation mechanisms of the wetting layer with different quench depths are discussed. The simulated results demonstrate that the growth of the wetting layer can exhibit pure diffusion-limited growth law, logarithm growth law,,,and Lifshitz-Slyozov (LS) growth law with the increasing quench depth, which reproduces experimental observations. Furthermore, the detailed ranges of these three regimes are determined on the basis of simulated results. The growth law of the wetting layer is pure diffusion-limited growth law when chi N < 2.01. In the case of 2.08 <= chi N, the growth of the wetting layer obeys LS growth law. However, when 2.01 <= chi N < 2.08, the logarithm growth law of the wetting layer is favored. The simulated results also demonstrate that the evolution of the polymer morphology in the parallel cross sections near the substrate surface obeys LS growth law. Moreover. the orientation effect of the surface on the dynamic behavior of these cross sections with deeper quench depth is more remarkable than that with shallower one.