Instantaneous features of signal can be used to identify the signal. A damage detection method for composite material is presented that uses instantane ous phase and an active monitoring method using Lamb wave. This paper firstly im itated a signal composed of normal and damage signals. Its instantaneous phase c hanges with damage emission. In succession, an experiment is conducted to get a series of signals before and after damage from the composite material by the act ive monitoring based on Lamb wave. The instantaneous phase changes when the comp osite material is damaged. The experimental results show that the instantaneous phase avails to structural health monitoring.