A method for developing a mathematical model of a large space structure with a discretely varying composition in the course of its in-orbit assembly is proposed. An umbrella-type antenna is assembled on the basis of a small deformable spacecraft. The structure skeleton is assembled from rods (uniform or hollow) with attachment points at the ends. The antenna reflector is assembled step-by-step as zones separated from each other by load-bearing rings. Each zone is assumed to be rigid and its mass-inertial characteristics relate to the corresponding characteristics of the deformable spacecraft. The load-bearing skeleton is supported by a net curtain forming the functional surface of the antenna. The method proposed allows one to develop a mathematical model of a large space structure at each stage of its assembly.