Abstract: Half-through truss bridges without upper lateral members are vulnerable to out-of-plane buckling of the upper chords. The instability modes caused by upper chords buckling involve obvious lateral bending and torsional deformation. The observed critical buckling load and buckling modes of upper chords are different with theoretical results. Based on FEA analysis, we analyzed some half-through truss bridges, and found that the stability is proportional to the width-span ratio. For those bridges, stability is directly proportional to its width-span ratio. The influence of the width span ratio on the stability was investigated by considering its influence on lateral stiffness and torsional stiffness. The influence of lateral stiffness is more significant, which affects the instability mode of the whole bridge. If the instability mode suddenly changes, the stability performance will change greatly. It is necessary to set effective link between the two main trusses of a bridge. Installing wind braces can effectively link the two main trusses, increase the lateral stiffness, and thus improve the stability, but the wind brace can not increase the torsional stiffness.