The Kaohsiung crossharbor tunnel was built using the caisson box sinking method and is the only Taiwan’s undersea tunnel. The construction of the tunnel faced extreme conditions such as sea salt which may result in corrosion of the tunnel structure, and also it is situated on weak seabed. However, after 20years of usage, the tunnel had begun to show slight damages, for example, joint corrosion, leakage and cracks in the tunnel etc. These damages to the tunnel may cause situation such as part of the joint displacement, structure inclination and causing detrimental effect to the overall structure of the tunnel, and hence, greatly reduce the tunnel’s ability due to earthquake. The tunnel is the main economic link of southern Taiwan. Therefore, the main point is to take into account all the reasons above, do an overall checkup on the tunnel and come out with a suitable proposal for any necessary reparation works. This report will explain matters related to the maintenance of tunnel joints, structure and other important points of a tunnel. It will also touch on the numerical analysis, the automated monitoring tools, and suggestion for any reparation works.