Abstract: For cable-stayed bridge without backstays under ideal completed stage, the in-plane instability of the structure many occur in the form of girder buckling. A formula based on energy method is proposed to calculate the critical buckling load of the main girder, adopting elastic foundation beam simplification and selecting the sine series as the displacement functions. According to the structural characteristics of this type of bridge, the simplified continuous functions are developed to describe the variations of the axial force of the girder and the equivalent elastic supporting stiffness of the cable. Moreover, the linear discount coefficient is introduced to account for the effect of pylon deformation on supporting stiffness. A case study on the critical load of Hongshan Bridge in Changsha is finally conducted for verification. The results are in good agreement with those of the finite element method.