A method based on wavelet packet energy spectrum is put forward to detect rail fastener loosening.Vibration signal is decomposed with wavelet packets

combined with energy variation in frequency domain before and after fastener loosening to build damage index（ DI）. On the above basis

conduct dynamic response tests of track structure under five conditions and identify and analyze on track structure state. Experimental results show that DI can effectively identify track structure state

which is significantly increasing with the increased degree of loosening; fluctuation range of DI under the same condition is within ± 0. 5

which proves that the proposed method has strong robust and capacity of resisting disturbance.