Abstract: The bending stiffness of segment joints, which has nonlinear characteristics, is related to their moment and axial force. The nonlinear characteristics of joints make tunnel's mechanism complicated. The bending stiffness of joints has great influence on internal force of lining through calculation. So, shell-spring-contact-ground model is established, among which the joint bending stiffness is adjusted according to the joint force. The real stress state of lining is obtained by iterative calculation. The model calculation results are verified through large scale laboratory tests. Then the model is used to analyze the force of Shiziyang tunnel lining in composite strata. The results show that the joint bending stiffness of iterative model is closer to the laboratory tests than constant stiffness model. For composite strata, the difference between iterative model and constant stiffness model is big, and it becomes even larger with the increased ratio of the soft layer thickness to the diameter of tunnel. When shield tunnel is set in homogeneous strata, the joint stiffness at each location of the whole-ring has little difference. If the appropriate values of joint stiffness is chosen, the constant stiffness model can also yield accurate results.