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Non-destructive testing has been used widely for the integrity testing of piles, especially low-strain testing methods, for example the sonic echo test, the impulse response method, etc. The length of a long pile is often difficult to evaluate correctly due to the fact that the energy of a reflected stress wave generally fades with the wave travel path during testing. The purpose of this paper is to use the three-dimensional amplitude and phase angle message of a complex continuous wavelet transform to determine the length of piles by analysing a time-frequency phase angle diagram. Five piles of different lengths were tested to verify the new approach used in this study. The testing results show that a complex continuous wavelet transform is not only able to provide high-resolution results in different frequency bands but can also simplify the identification of the refection point using the three-dimensional phase angle spectrum. The pile length and pile tip condition can be easily determined using a phase angle diagram under the corresponding frequency of maximum amplitude in the three-dimensional amplitude spectrum. [ABSTRACT FROM AUTHOR]