Abstract: The internal force of statics system， namely the force and the reacting force， occurs in pairs and involves two objects. When the principle of virtual displacement is used to solve the internal force of a static mechanical system， the forces and the reacting force generally have virtual work， it is necessary to determine the virtual displacement separately and calculate the virtual work separately， and which is a complex process. In the report， reconstructing the external constraints of the system and fixing one of the two objects involved in the internal force of the system was proposed. Therefore， the virtual displacement of one force in the force and the reacting force is zero and the virtual work is zero too， which simplify the calculation process of internal force virtual work. When dismantling the original constraints of the statics system， the corresponding constraint forces should be retained. When rebuilding the system constraints， the system cannot become statically indeterminate.