The classification of system running state is determined and the transformational relation among them is studied. State equations are constructed about simple single generator-load model system. Steady state and the state of emergency running are focused. The occurrence of a large disturbance in a power system can arouse active and reactive power imbalances. System frequency and voltage trajectory are simulated and analyzed with Matlab. The results show that the frequency is an indicator of real power balance and reactive power imbalance determines the change of bus voltage. Frequency and voltage stability cannot be decoupled completely under emergency situations. This conclusion has an important guiding significance for the design and improvement of traditional under frequency load shedding (UFLS) strategy.