Abstract: For straight beams with a constant cross-section that are subjected to concentrated forces, concentrated couple moments, and uniform load, a simple method for rapidly calculating the strain energy based on the geometric properties of the internal force diagram is proposed. The strain energy is expressed as the outcome of a simple arithmetic operation of geometric parameters determined by the shape of the internal force diagram. The example demonstrates that this method avoids the formulation of the structural internal force equation and other intricate computations, simplifies the process of calculating the strain energy, and enhances the calculation efficiency.