This article analyses the theories of two methods of matrix displacement and finite element, and the application of calculation in the actual structure. Then it explores the subtle differences between them: Matrix displacement method can only be used for analysis with known unit nodes force-unit nodes of the relationship between the displacement of the truss structure, and not the continuous body structure. The major differences between both are the establishment of the basic principle and solution of the emphasis. Establishing total stiffness matrix according to the geometric property of the structure and then forming integral force vector via loading conditions when matrix displacement method is used to solve the unit fixed-end force. It’s necessary to stack fixed end force; While finite element method mainly bases on the known load and the material nature of structure after structure discretization. Node force or node stress is directly got by the derivation of displacement, but the fixed end force is lost, which dropping the accuracy of the stress. To solve plane frame structure and get structural internal force, the distinguish is checked on the example.