The truss string structure, composed of lower string, upper truss and struts, is one kind of the long span pre-stressed steel structure. During the pretension procedure, the struts are compressed since the tension of the string. At the same time the inner force and deformations, which are opposite to the effect of outside load, appear in the truss. The structure stiffness and integrity are improved. Before pretension, lots of falseworks are set up in order to supply the temporary support for the upper truss. The reasonable pretension of the string commonly conforms a principle that the upper truss is just moved up from the falseworks. And meanwhile the predefined tension force is reached. Therefore, for getting a reasonable pretension method and pretension force, it is important to precisely simulate the coaction of the elastic falseworks and the truss. A actual truss string of a roof structure is taken as a comparison model for pretension simulation and ANSYS program is used in this paper. Detail analysis models are showed as Fig 1 and Fig 2. Different simplifications are used to simulate the coaction of the falseworks and the truss. After comparison of the results, an reasonable simulation model and a tension control method are obtained. The reliable pretension method and optimal pretension force are provided. It is valuable for future similar engineering.