The dynamic method for identifying axial force is grounded in vibration theory， making the vibration equation of a bar member crucial for accurate axial force estimation. Traditionally， the Timoshenko beam is derived from the equilibrium of trans? verse forces and moments. In this paper， an energy-based approach is applied to derive a new vibration equation for the Timoshen? ko beam under axial loading. The Ressiner energy equation for a Timoshenko beam， incorporating displacement， stress and axial force， is established using a condensation hypothesis from an energy perspective. The motion equation and stress equilibrium are calculated using the extremum principle， leading to a new free vibration equation for the Timoshenko beam under axial force. Com? pared to classical textbooks， the proposed dynamics equation includes two additional terms related to axial forces and shear effects. The new equation is validated through numerical simulations and laboratory experiments to identify the axial force in bar members. The results demonstrate that the proposed equation significantly improves the accuracy of axial force identification， confirming its correctness and applicability.