In order to determine the optimal force system of a large-span spatial structure (LSSS)，based on the morphological knowledge and basic theory of ESO method，the three-dimensional solid model of an initial design domain was established by ANSYS software；and by taking the status of every unit as the design variables and the minimum volume as the objective function，the ESO method was then applied to the LSSS for structural force system optimization.To meet the architectural intent and the restrictions of special foundation conditions，the arch-bearing force system of steel structure was finally presented.Eventually，by comparing the static analysis results of the deformation and inner force for structural force systems determined by the ESO method and original inverse hanging method，the feasibility and availability of the ESO method was demonstrated.It is shown that the topology optimization methods are essential and necessary for optimizing structural force system of the LSSS.