China possesses a vast scale of in-service tunnels. With their increasing service life, ensuring the operational safety of lining structures has become a critical industry concern. The frequent occurrence of lining spalling incidents in recent years has resulted in substantial economic losses. This issue is compounded by a lack of effective detection methods and the limitations of conventional techniques for assessing existing tunnel lining defects, making the early identification of loosened lining segments an urgent demand for practitioners. A non-contact detection method was proposed for tunnel linings that integrates a directional acoustic wave generation device with laser Doppler vibrometry technology. Model tests were conducted to obtain the vibration characteristics of tunnel linings containing loosened segments. Findings indicate that the presence of loosened lining segments significantly reduces the overall structural stiffness while simultaneously weakening the local stiffness at the section containing the crack. These characteristics manifest in the monitoring data as: a sharp drop in the dominant vibration frequency at local measurement points. Significant fluctuations in the rate of change of the spectral kurtosis area of vibration signals between the loosened segment zone and the intact zone.These features that are sensitive to potential spalling locations can be effectively used for the accurate localization of potential spalling blocks, thereby further promoting the development of non-contact identification technologies for potential tunnel lining spalling.