Abstract: Unit-load method is a general method to solve displacement, which plays an important role in the energy method of material mechanics. When using virtual displacement mode in the unit-load method, the main limiting condition for virtual displacement is to satisfy the displacement boundary condition. For statically indeterminate problems, especially for the unit-load system construction with free boundary conditions, the understanding of the limiting condition determines the flexibility of using the unit-load method. In this paper, several classical examples are used to analyze the effect of this restriction, and to provide reference for explaining related problems in teaching activities.