Abstract: 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.