Abstract: In order to investigate the bending state nonlinearity of shield-tunnel ring joints under the composite action of longitudinal internal forces, the mechanical behavior of ring joints were classified to 7 deformation modes and 3 internal force states. The analytical expressions involved ring joints rotation angle and longitudinal internal forces for each deformation mode were derived. At the same time, the critical moment expression of the ring joints between different deformation modes was developed. FEM method was employed to verify the results obtained by an analytic derivation. The results showed that bending state nonlinearity was one inherent feature of ring joints because of the discontinuous structure of a shield tunnel. The deformation mode and bending stiffness of shield-tunnel ring joints changed under different internal force combinations. In a pure bending state, the ring joints' bending stiffness had a constant value, and it could be enlarged several times under a compression-bending state, even be enlarged to infinity. The bending stiffness of ring joints under a tension-bending state would be limited to two constant values which were both smaller than that under a pure bending state, and it would be decreased several times when ring joint surface was completely opened.