Abstract: This paper uses the work-energy theorem to solve the small deformation problems for a liner elastic bar. According to a geometric analysis of the deformations of a simple linear elastic bar, the work-energy theorem is found to be more effective for the purpose. A force H perpendicular to the loading force P is assumed to be applied to the bars before the loading of P, and based on the work-energy theorem, we can obtain the formula to calculate the displacement caused by P. This methodology can be extended to more complex elastic bar system such as a truss and the computer programming will be easy, which can greatly simplify the calculation of the small deformation of a truss.