Chaotic oscillation is an inherent phenomenon of nonlinear power system,which is very harmful for the large-scale interconnected power grid. Based on the three-node power system,a seven-order mathematical model of this system is deduced in this paper. The dynamic characteristics of the three-node power system are analyzed through bifurcation diagrams and phase portraits,and the influence of system parameters varying on the operation states are studied as well. Then the electromagnetic power disturbance and the load power disturbance are introduced into the system model,which makes the model closer to the actual situation.The changing process in dynamic behaviors of the system under the impact of the disturbance amplitude and frequency are illustrated with bifurcation diagrams and phase portraits for specific parameters are given respectively. Moreover,it is found that the operating state of the system can reach periodic motions from the chaotic motions when the amplitude and frequency of perturbation terms are within certain ranges.