A general analytical model applicable to flexural–torsional coupled vibration of thin-walled composite box beams with arbitrary lay-ups under a constant axial force has been presented. This model is based on the classical lamination theory and accounts for all the structural coupling coming from the material anisotropy. Equations of motion are derived from the Hamilton’s principle. A displacement-based one-dimensional finite element model is developed to solve the problem. Numerical results are obtained for thin-walled composite box beams to investigate the effects of axial force, fiber orientation and modulus ratio on the natural frequencies, load–frequency interaction curves and corresponding vibration mode shapes.