The force-deformation curve of a reinforced concrete structure in push-over analysis is used to compute the corresponding damage index. A simplified and practical method is established for seismic damage assessment in combination with capacity spectrum method. The two-dimensional capacity spectrum method and the combination of the damage in two directions are discussed. A numerical example using single and double horizontal capacity method and time history analysis demonstrate that the response and damage of structure can be computed accurately in the two-dimensional capacity method. The common trend of structural torsional response can also be reflected. The results show the two-dimensional capacity method is valid and simple.