In order to diagnose the damage of longspan spatial steel structures, a three-stage damage diagnosis approach was proposed. In three stages, the potentially damaged regions, potentially damaged elements, and damage severity were successively determined. A flexibilitymatrixbased damage index FDI (flexibility difference index) was developed to determine damage regions, the damage locating vectors method was utilized to locate damaged elements, and the secondorder eigensensitivity analysis was employed to estimate the damage severity. Then according to the results of elastoplastic time history analyses of the steel roof structure of National Stadium, three damage patterns were simulated. Furthermore, numerical simulated analyses of damage diagnosis of the steel roof structure of National Stadium was carried out. The result showed that the threestage damage diagnosis approach could effectively diagnose the seismic damage of longspan spatial structures.