Time reversal is the process that a response signal recorded at a receiver location is reversed in time and emitted back to the original source transducer. In the absence of any damage, theoretically the main wave packet of the reconstructed signal could exactly resemble the original input wave form. However, since the amplitude response of Lamb wave mode is generally frequency dependent, the reconstruction is not perfect. In this study, the influences of the frequency-dependent amplitude to the spectrum of the reconstructed signal is investigated. The results show that the peak frequency shifts and the bandwidth varies due to the frequency dependency of amplitude. This spectrum distortion affects the wave form of the main wave packet of the reconstructed signal significantly. Since the baseline-free damage detection is accomplished by comparing the wave form of the TR reconstructed signal with that of the original input signal, these effects would be captured by the damage index and be attributed to the presence of damage. Thus a false alarm may arise. To mitigate these effects, an amplitude modification strategy is established, and an experiment example is introduced to illustrate its process. With the application of this strategy, the modified baseline-free damage detection method is then employed for monitoring the growth of damage.