Multi-layer dielectric structures mostly exist in road structures,ballastless track structures,tunnel structures,etc.and their internal defects directly affect the safety and durability.The existing ultra-sonic SAFT imaging method is only suitable for the detection of single-layer structures.Based on the ultra-sonic SAFT imaging method,the ultrasonic travel time is accurately obtained through the reverse time cal-culation model,and the imaging detection of multi-layer concrete structures is realized.The experimental results show that the ultrasonic SAFT imaging method based on the reverse time calculation model can ac-curately detect the defect position in the multilayer dielectric structure,which provides a theoretical basis for the imaging detection of the multilayer dielectric structure.