Abstract: In the nonlinear responses analysis of reinforced concrete structures, seismic wave input angles are usually simplified as paralleling to the principal axis of structures, and the problem of critical excitation angle is generally ignored.In this paper, three regular and symmetrical reinforced concrete typical spatial frames were employed as examples.Based on the OpenSees framework, flexibility method based fiber model was adopted to simulate the nonlinear responses of the columns under biaxial bending and varying axial forces.Nonlinear dynamic response analyses of the three frames under rare earthquake excitations with varying seismic wave input angles were carried out.The results show that for the regular and symmetrical spatial frames, the critical excitation angle of compressive strain of concrete fibers, tensile strain of steel fibers and ratio of plastic hinges at column ends are 45 and 135, the critical excitation angle of strain of concrete fiber and steel fiber, ratio of plastic hinges and rotation ductility at beam ends are principal axis of structures.The critical excitation angle of rotation ductility at column ends is not apparent.It cannot get critical energy dissipation characteristics of plastic hinges if seismic wave was input along the principal axis direction.