Non-destructive testing of structure with the change of vibration characteristics attracted much attention in recent years. The basic idea of this method is that the vibration characteristics can reflect the state of the structure, and damages can cause changes of bridge vibration parameters. In order to study the method detecting steel bridge damages based on the power spectral density (PSD) of acceleration response, a disused railroad bridge in Japan was adopted as specimen, and six cases of damage were introduced to it in advance. The bridge was actuated by sinusoid sweeping frequency wave, and then the power spectral density differences between the undamaged and damaged bridge were analyzed. Experimental results show that the steel bridge damages can be detected and located by the changes of acceleration PSD.