Focusing on the application with diagonal braces, tie-bars and short thick cables, this work proposes an analytical method to identify the absolute axial force of a beam member with uncertain boundary conditions. Bending stiffness effects are taken into account. Based on the dynamic measurements, the natural frequencies and the modal displacements／strains of the beam can be extracted to construct a characteristic equation for the estimation of the axial force. Finite element simulations verify the theoretical formulation, accounting for various sensor placements, various utilizations of modal information and various boundary conditions. A lab experiment also confirms that the proposed method can identify the absolute axial force of a beam with high precision by separately using the first five modal parameters.