Abstract: The out-of-plane stability of a bending beam, which is made up with 2 beams and loaded by concentrated vertical loads, is studied. Differential equations of each beam with both bending deflection and torsinal deformation is established in critical condition. According to compatibility conditions of deformation, the characteristic equation is obtained for out-of-plane stability of the cantilever bending beam. By analyzing bending beam consisting of 2 similar beams, the stability characters of upwards-bent and downwards-bent beam are discussed. The effectof torsional stiffness on stability is studied. Under the same load and beam length, downwards-bent beam has higher lateral stability than straight beam, but the upwards-bent beam has lower lateral stability. There exists a best bending angle to obtain the highest stability. The increase of torsional stiffness will lead to the increase of the lateral stability for upwards-bent beam, but reduce the lateral stability for downwards-bent beam.