In order to achieve a lightweight structure of the bus frame consisting of rectangular tubes,first,the cross-sectional properties of the rectangular tubes are calculated,and the responses of static bending stiffness,static torsional stiffness，and dynamic frequency stiffness are respectively defined.Next,a structural optimization model of the bus frame is constructed.In the model，the objective function is the structural mass,the design variables are the cross-sectional breadth,height,and thickness of the rectangular tubes,and the constraints are the bending,torsional and frequency stiffness.Then,the sensitivity information of the responses with respect to the design varia- bles is derived,and the nonlinear optimization model is solved through the sequential linear programming.Finally， by a numerical example,it is proved that the proposed method can efficiently acquire the optimal bus frame that satisfies the constraint conditions.