A general analytical model based on shear-deformable beam theory has been developed to study the flexural–torsional coupled buckling of thin-walled composite beams with arbitrary lay-ups under axial load. This model accounts for all the structural coupling coming from the material anisotropy. The seven governing differential equations for coupled flexural–torsional–shearing buckling are derived. The resulting coupling is referred to as sixfold coupled buckling. Numerical results are obtained for thin-walled composite beams to investigate effects of shear deformation, fiber orientation and modulus ratio on the critical buckling loads and corresponding mode shapes.