Abstract: A new simplified analytical model, namely Elastic foundation—Spatial structure member—Storey model, is developed for free vibration analysis of large spatial frames of high-rise buildings built on soft soil. Only axial rigidities are considered for columns of the subsidiary frames. Large spatial frames are then idealized continuously as Winkler elastic foundation beams along the axes of the girders. A large spatial frame supported on semi-infinite elastic foundation is then decomposed into a frame with large columns and girders with a Winkler foundation beam underneath. The stiffness matrix used for the free vibration analysis is thusly obtained through the simplified frame; the masses of the frame are considered to be concentrated on its floors. Free vibration analysis is then performed. Examples show that the computing model is rational and feasible.