Abstract: A beam string structure (BSS) loaded in a plane may suddenly buckle in its out-of-plane for the absence of lateral bracings. The out-of-plane stability of a BSS is classified as the stability of a string-struts system and the global stability of a BSS. Firstly, the equilibrium procedure and energy method are used to investigate the stability of a string-struts system. After that, it is pointed that the current design method is not safe for analyzing the global stability of a BSS, which takes string and struts as a concentrated load acting on the beam of the BSS. By using the principle of stationary potential energy, the potential energy equation of a BSS is set up and Ritz method is used to investigate the out-of-plane elastic buckling load of the BSS due to a uniformly distributed load or a uniform moment respectively, and the results obtained agree very well with finite element method (FEM) results. Therefore, the out-of-plane buckling load of a BSS proposed has reference value on its out-of-plane stability design.