Composite materials have been widely used in aerospace manufacturing because of their low mass, high specific strength, high specific stiffness and good fatigue resistance. However, composite structures are susceptible to damage such as debonding and delamination due to various impacts, vibrations and other external loads. Therefore, it is crucial to regularly monitor composite structures in real-time during spacecraft service to ensure the safety and reliability of the structures. Ultrasonic guided wave is an effective means of monitoring the health of composite plate structures. Delamination, debonding and other damages of composite materials can be imaged and localized using ultrasonic guided wave signals and probabilistic imaging methods. However, due to the large damping of composite materials, the traditional probabilistic imaging method has the problem of low accuracy in damage localization. Therefore, in this paper, we propose a probabilistic imaging method based on a modified time reversal for damage localization and imaging of delamination damage. The proposed method was experimentally validated using a composite plate as a test piece and compared with the conventional method. A fully automated falling hammer impact tester was used to create low-velocity impact damage on specimens to compare the localization accuracy of the time-free reverse method and the modified time reversal method for delamination damage. The results show that the modified time reversal method can better localize and image the delamination damage of composite structures with higher localization accuracy and more sensitive to damage, which verifies the feasibility and accuracy of the method in the localization of debonding damage of composite structures.