In order to investigate the yielding mechanism, spread of plastic area and collapse mechanism of tri-directional beam string structures(TBSS), the elastoplastic ultimate capacity of the TBSS of a real project was analyzed with the finite element method. The effects of connection condition of arch root, cross-sectional moment of inertia of arches, areas of cable and pretensioned force of cable on the ultimate capacity were also studied. The results showed that this TBSS has good performance of inner force redistribution and the collapse of the whole structure is due to material damage. The ultimate capacity has small relation with the connection condition of arch root, but the pin-ended should be used for the arch roof from the collapse mechanisms of the whole structure. And the ultimate capacity can be enhanced by increasing the cross-sectional moment of inertia of arches. In order to prevent first failure of the cable, the proper cable area and pretesioned force should be chosen.
