Abstract: For prismatic straight beams subjected to concentrated forces, concentrated couple moments, and uniform load, a simple method is proposed for rapidly calculating the strain energy based on the geometric properties of internal force diagram. The strain energy is expressed as the result of simple arithmetic operations using geometric parameters derived from the shape of internal force diagram. An example illustrates that this method eliminates the need to formulate the structural internal force equation and other intricate computations, simplifies the process of calculating the strain energy, and improves computational efficiency.