Abstract: The authors choose a single intersection of four phases as a model to put forward a new optimal time-planning scheme for traffic light based on the model of hybrid automata for single intersection, by using method of optimization for hybrid systems. Fixing period, the average queue length over all queues in one period is regard to objective function to find an optimal switching scheme and optimal order of phases for traffic light The method based on hybrid system can be used to compute the optimal order and optimal switching instant of phases.