In this study, signal processing approaches and nonlinear indicators are used to measure seismic responses of reinforced concrete structures. To analyze structural nonlinearity, an equivalent linear system with time-varying model parameters, singular spectrum analysis to elucidate residual deformation, and wavelet analysis to yield the energy distribution among components are adopted to detect the nonlinearity. Then, damage feature extraction is conducted using both the Holder exponent and the Level-1 detail of the discrete wavelet component. Finally, the modified Bouc-Wen hysteretic model and the system identification process are employed to shaking table test data of the RC frames to evaluate the physical parameters, including the stiffness degradation, the strength deterioration and the pinching hysteresis. The stiffness and strength degradation of the RC frame in relating to the degree of ground shaking and damage index are discussed.