Abstract: In the ultrasonic phased array structural health monitoring method, the damage scattering signal gotten by comparing the damage structural response signal with the health structural response signal can be affected easily by some external factors, such as, environment and the change of the structure. In order to solve this problem, the baseline-free phased array damage identification imaging method was proposed. The window function was used to get the damage scattering signal and to exclude the direct propagation wave and the reflection wave from the structural boundaries. Based on the phased array theory, the response signals travelling in the damage structure could be used to realize the damage recognition, and the health structural response signals no longer act as the reference signal in the calculation procedure. Experiments on the carbon composite structure show that the damage in the structure can be recognized precisely by using the baseline-free phased array method. The method does not need the baseline signal. It can greatly shorten the monitoring time and is not affected by the change of the environment and the structure. This research is the foundation of the engineering application for the method.