Abstract: "The Energy Method for Displacement Calculation" is a distinctive chapter within the material mechanics curriculum. In teaching, to circumvent the concepts of dual variables (and space), external potential energy, and the principle of virtual stress, and given that no textbook positions the Principle of Virtual Displacement as a fundamental principle for application, the logical connections within its content are not clearly articulated. In view of this, this article summarizes and discusses the relationship of the Castigliano Theorem, the Complementary Energy Theorem, the Reciprocal Work Theorem, the Reciprocal Displacement Theorem, the Principle of Virtual Displacement, the Unit Load Method, the Graph Multiplication Method, and the Regularization Equation of the Force Method. The progressive hierarchy among these contents has been clarified, and the framework of the knowledge system has been presented.