As the barely invisible damages are often induced during the manufacturing processes or in-service applications, the material strength will degenerate and the performance of structure will decrease. Lamb wave based damage detection method can be used to identify the damages effectively. However, due to the complicated amplitude modulation, the distribution of energy spectrum will shift which brings challenge to group velocity determination. As the group velocity often determined by the center frequency of the excitation, the error will be induced when the distribution of energy spectrum is changed. In this paper, on the basis of amplitude modulation curves, its influence on energy spectrum shifting is investigated through simulation and experiment. Furthermore, the group velocity correction method is proposed to improve the damage localization accuracy. The experiment is carried out on a T300/3231 composite laminates. The imaging result shows higher accuracy when using corrected velocity, which verifies the efficiency of the proposed method.