At present, the mainstream method for treating the large deformation defects in subway shield tunnels is steel plate superimposed reinforcement. However, the repair design merely stays at the level of structural reinforcement, and there is no clear calculation method for the bearing capacity and stiffness of the lining structure after the reinforcement. Therefore, this article establishes a calculation method for the mechanical properties of shield tunnel reinforcement based on the failure mechanism of superimposed reinforcement, and the method has the following characteristics: (1) It continues the conventional design theory for correcting shield pipe segments, and the calculation method for mechanical properties is based on the force method principle and the failure mechanism of the reinforced shield tunnel; (2) Based on the macroscopic mechanical properties of the segment and the reinforcement materials, a formula for calculating the section stiffness under both positive and negative bending moments was established. During the calculation process, an iterative matching between the section stiffness function and the sign of the bending moment ensures that the value of the section stiffness corresponds to the sign of the bending moment; (3) Taking into account the strain lagging phenomenon of the reinforcing materials in the composite lining structure, the development history of the section strain is also considered; (4) The initial deformation of the large deformation shield tunnel to be reinforced is considered through a radius function. The reliability of the calculation method is verified by comparing with full-scale test results. Based on the existing standards, the repair design process is divided into two types: control by bearing capacity and control by deformation. At the same time, a reinforcement design example is presented.