Abstract: For the plane triangular truss structure, the assumption of small deformation in materials mechanics can be used to simplify the solution process by replacing arcs with tangents, to meet the actual needs of the project to a certain extent. The knowledge for the application scope of this small deformation hypothesis is important for students. In this paper, taking the plane triangular truss structure as an example, the theoretical derivation and the finite element analysis are used to quantitatively analyze the applicable range of the small deformation hypothesis of the typical structure through a specific calculation example. The derivation process and the analysis are generalizable, which can help students understand the application of materials mechanics assumptions in engineering practice.