Abstract: Due to the particular and complex engineering environment of subsea tunnels, it is especially imperative to monitor the condition of steel corrosion in reinforced concrete (RC) of subsea tunnels for structural serviceability and durability of RC structures. In this paper, the largest excavation section subsea tunnel-Xiamen Xiang'an subsea tunnel is studied. Especially, the design of corrosion monitoring system is focused due to its complexity. Both the commercial corrosion sensors and the developed corrosion sensors based on electrochemical and fiber Bragg grating (FBG) techniques are adopted. The arrangement of the corrosion monitoring sections, their location, the installation of the sensors and the establishment of corrosion database are discussed. Then, the evaluation of corrosion in subsea tunnel is established based on the monitoring data of reinforced concrete collected in field. Based on the monitoring data collected in the field of the subsea tunnel, the status of stress and corrosion in Xiamen Xiang'an subsea tunnel can be evaluated. Finally, the promising technique based on ultrasonic guided waves for monitoring steel corrosion in RC is also discussed.