Abstract: On the basis of cable net optimization，shape-finding and initial prestress determination，by taking the lowest structure weight as objective function， an optical model of two-way bean string was established，which includes three levels cross-section optimization of cable ，top chord and secondary member such as strut． Stress constraint， local stability constraint and displacement constraint were all considered on this optimization model． Then a two-step and three-level optimization design method of the beam string was raised by the APDL program on ANSYS optimization module． The example shows that the result of this optimization method can be converged fastly，which can be used for initial design of practical projects．