Using a nonlinear quasi-geostrophic barotropic model, the paper investigates numerically how the spatial pattern of synoptic scale waves and the westerly wind strength affect the phase transition of the North Atlantic Oscillation (NAO), and the role played by the large-scale topography. It is found that the weak basic westerly wind is favorable for the westward shift of the downstream large-scale system, favoring the transition of the NAO from positive (negative) phase into negative (positive) phase. It is noted that when the synoptic scale waves are stronger and concentrated in a wider region, the NAO event can transform more easily from the negative phase into positive phase; when the synoptic scale waves are stronger and shift northward, the NAO can transform more easily from the positive into negative phase. In the presence of a large scale topography, the number of the cases in which the NAO transforms from positive (negative) phase to negative (positive) phase is reduced.