By monitoring the structural vibration and analyzing the modal parameter, hidden damage can be identified at an early stage, preventing abrupt failures for important structural components. However, the modal parameter has exhibited a low sensitivity to the interfacial damage, such as the delamination in the composite beam. In this paper, a signal-processing method is developed to increase the sensitivity of the modal parameter to the delamination. Both the first- and high-order modal parameters are transformed to an approximate waveform capacity dimension (AWCD). Based on the wave peaks of the modal parameters in the AWCD, the start and the end of the delamination can be accurately pinpointed. Since the AWCD method can highlight the delamination-related waveform features of the modal parameters, the delamination can be identified without referring to the baselines. Both analytical and experimental investigations demonstrate that the AWCD method can modify the waveforms of the modal parameters and lead to an accurate identification of the delamination in composite cantilever beams.