Power system resonance occurs in cases of strong non-linearity. This paper uses the simplest normal form theory to derive the simplest normal form of a power system at the second-order resonance point. This approach is able to solve the problem that traditional normal form coefficients at the second-order resonance point are strange. By calculating the approximate analytical solutions，participation factors can be fixed，the changing process of the dominant stability mode can be analyzed;then the physical mechanism caused by the second-order resonance point dominant stable mode can be revealed. The simulation results indicate that the second-order resonance close to the saddle-node bifurcation is the critical region which forms the voltage stability dominant mode. This change is caused by the resonance terms. The proposed method is useful for a profound understanding of the relationship between voltage stability and angle stability.