As a typical marine and underwater infrastructure, the joints stress state of immersed tunnels is complicated due to the differential settlement during operation. Based on the current attitude of the segmental elements, this study takes GZ Tunnel as an example to preliminarily discuss the influence of differential settlement on the mechanical behavior of immersed tunnels through field monitoring, on-site detection and numerical simulation. The result shows that: The differential settlements of immersed tunnels can be divided into longitudinal and transverse, specifically manifested as bending or torsion of elements. The maximum longitudinal settlement and maximum transverse settlement of immersed elements is 108.7 mm and 27.9 mm, respectively. Under the influence of differential settlement in different directions, and the shear keys of the central wall and the sidewall with larger settlements are subjected to greater forces. Numerical simulation shows that the maximum principal stress and minimum principal stress of GZ Tunnel can reach 20.87 MPa and 22.21 MPa, respectively, which is located at the joint between E2 element and the northern buried element. The failure mode of the shear keys is cracking near the inner corners and local concrete crushing. The failure reason can be summarized as the compressive or tensile shear caused by the misalignment and torsion of the elements. The reason of differential settlement of GZ tunnel is caused by construction and long-term operation. In studying underwater facilities such as immersed tunnels under the influence of differential settlement, it is suggested to consider the three-dimensional attitude change of the structure comprehensively.