At present, tunnel leakage is mainly discovered by manual inspection, which seriously affects the accuracy of leakage identification. The feasibility of using infrared thermal imaging to detect tunnel leakage is explored, with the Outer Ring Tunnel in Shanghai, China, chosen as a case study. The Outer Ring Tunnel belongs to an immersed tube tunnel. In site experiments are performed to detect leakage under operation environment. In summer, the groundwater temperature is lower than that of the concrete surface, causing the infrared images of the tunnel leakage area to show low-temperature zones surrounded by high-temperature regions. When the ambient temperature is low, the temperature difference between the tunnel defect leakage point (the lowest temperature point) and the intact wall (the highest temperature point) is 2.6 ℉ ~ 3.3 ℉. When the surrounding temperature is elevated, the temperature difference ranges from 3.6 ℉ to 5.1 ℉, which shows that infrared thermal imaging detection of tunnel leakage is feasible under natural conditions.