Abstract: Planar BSS with top chords of H-section can be made conveniently,and its joints can be connected easily,but the system is sensitive to the imperfection out of plane because the member of H-section has small stiffness out of plane.The system will be unstable if no effective measure is taken in construction,which tells failure of construction.The paper builds spatial model of the system by FEA and analyzes the stages of prestress-tension and use,researches the spatial results brought by lateral supports,gives out rational scheme of construction,and points out the key problems,which ensures the safety of construction of BSS and can be a reference for other similar projects.