Non-destructive testing (NDT) has been used widely for the testing of pile quality, especially in low-strain testing methods, e.g. the sonic echo test, the impulse response method, etc. The length of a long pile is often difficult to be evaluated correctly due to the fact that the energy of a reflected stress wave generally fades with the wave travel path during the testing. The purpose of this paper is to study using a signal processing approach to detect unknown pile length, especially for a long pile. This approach successfully utilized the amplitude and phase message of a complex continuous wavelet transform to determine the pile length of the pile by analyzing a time-frequency-phase angle diagram in the different frequency band. Two long drilled piles were installed and tested to verify the proposed approach in this study. The testing results show that a complex continuous wavelet transform is able not only to provide high-resolution results in different frequency bands but can also simplify the identification of the reflection signal of the pile tip using three-dimensional phase angle spectrogram. The length of the pile can then be easily calculated using the time of wave traveling from the pile top to the pile tip.