The quadratic optimal control for a class of switched systems with step perturbations was studied, in which the switched sequence and switched number of the system was fixed. By means of dynamic programming principle, a kind of multistage decision processes was proposed, which could obtain the optimal switched time and the optimal control input. The improved genetic algorithm(GA) was utilized to search the optimal time and the optimal control input. A numerical example was given to illustrate the effectiveness of the design approach.