Prestressed space grid structures have been widely applied in long span structures for many years. The key problem of such structures is how to control the prestressing force during the pretensioning process. An initial internal force (IIF) method for pretension process analysis of prestressed space grid structures is proposed in this paper. With this method, the control force of a prestressed member can be calculated. By pretensioning members to the control value, the final force of prestressed members, which is generally designed before pretension, will be obtained successfully. Furthermore, the internal force of unprestressed members and the nodal displacements can be obtained easily when the structure is pretensioned by different schemes, such as one member is assembled and pretensioned, several members are assembled and pretensioned synchronously, or members are pretensioned step by step. During the computation, the global stiffness matrix of the structure is assembled once and can be employed directly or with some local modifications under different pretension conditions. The computational rate is thus greatly improved. The numerical results show that the proposed method is correct, reliable and effective and can be applied to linear analysis of prestressed structures.