Abstract: PSD multi-parameter approach is often used in concrete pile quality rating. However, in testing process, because of random disturbance to the inspection signals and discrete features in the concrete pile, singular signals may be produced and lead to miss or misinterpretations of defects. In this paper, suspected defect signal was decomposed by wavelet transform, and the disorder of signal was quantitatively described by introduction of approximate entropy. The results showed that under the wavelet decomposition, the amplitude of defect signal was attenuated in the high frequency band with a clutter waveform and larger approximate entropy. The present method was be fitted for confirming suspected defect and showed good effectiveness in the practical pile foundation inspection.