Abstract: Grouting defects behind the lining pose a significant threat to the safety and durability of service tunnels. Timely and precise detection of these defects is crucial for extending the tunnel structure's service life and ensuring its operational safety. Therefore, a rapid detection method using rotationally excited air-coupled impact-echo was proposed to address the low efficiency of contact-based point-by-point testing in conventional tunnel inspection. A numerical model of grouting defects behind the lining was established to analyze the response characteristics under air-coupled excitation. A rotational air-coupled impact-echo detection system was then developed, and physical model tests were conducted on grouted linings with multiple defects. A defect imaging index, Response Ratio Amplitude (RRA), was introduced and defined as the amplitude ratio between the frequency corresponding to the overall grouted thickness and that of the single-layer lining. We found that the air-coupled method effectively captures the impact-echo response, and the defects result in the disappearance of dominant frequency components associated with the overall lining thickness. The proposed system achieves more than a tenfold increase in detection efficiency compared with manual point-by-point methods. The RRA index enables accurate imaging of defect locations and sizes, with detection accuracy exceeding 85%. This approach provides theoretical and technical support for rapid, non-destructive detection of grouting defects behind tunnel linings.