In order to improve design optimization efficiency and quality for solid rocket motor, a small SRM was optimally designed with the impulse weight ratio as the objective by using genetic algorithm. This algorithm adds a reverse operation and combines with penalty function method to deal with the inequality constraints. The calculation results shows that in a given space, the genetic algorithm can obtain the optimal solution of the design variables quickly and effectively, and the impulse weight ratio is raised. It provides an effective method of optimization design for engine designers.