In order to study the relationship between the nonlinear dynamic characteristics and the impairing degree of reinforced concrete U-shaped beam， a static and dynamic testing on a simply supported reinforced concrete U-shaped beam with a scale ratio of 1∶10 was conducted in this paper. The static loads on the beam caused different degrees of damage. At all levels of damage load conditions, dynamic test on the U-shaped beam were conducted and the acceleration-time signal were collected. Finally, the filter and the EMD were applied to decompose the first and the third order frequency signal of each state. Then the Hilbert transform was used to identify the first and the third order amplitude-time curve and frequency-time curve of each state, and the frequency-amplitude chart and normalized frequency-amplitude comparison charts were obtained. By analyzing the curves, the relationship between nonlinear dynamic characteristics of the reinforced concrete U-shaped beam and its damage levels were obtained. The structural nonlinear dynamic characteristics can be used to detect damage for those structures which needed high integrity. But for the common U-shaped beam or other structures, only using the nonlinear dynamic characteristics to detect damage was unreliable, the structural nonlinear dynamic characteristics can be regarded as a means to warning damage or to be used with others detection method.