In order to seek the reasonable design method of bidirectional beam string structure, the effect of main geometric parameters on steel consumption was analized. By using the optimization design method of bidirectional beam string structure based on equilibrium load state, the minimum amount of structural steel as the optimization target, the influence to the structure optimization resulted by the high-span ratio, sag ratio, spacing ratio in two directions aspect ratio, the balancing load value and other parameters was discussed. It’s found that the high-span ratio and the sag ratio increasing lead to linear descending of steel consumption. The low influence by spacing ratio in two directions makes the structure space arrangement of ambassador can be flexibly handled. The balancing load has an optimal value and has a big effect to the structure stiffness on load state. Bidirectional beam string structure design should be preferred to increase the high-span ratio and try to increase the sag ratio. Prestress is determined by balancing load caculation.