Abstract: With the rapid development of tunnel construction, cased of new tunnels undercrossing existing tunnels are increasingly common. Accurate prediction and control of longitudinal settlement is critical for structural safety. The beam-spring models assume a constant subgrade reaction modulus k, which poorly suited for scenarios like large deformations in weak clay. This paper proposes a method for predicting longitudinal settlement based on Mobilized Strength Design (MSD), focusing on the nonlinear relationship between subgrade reaction and settlement. Applying the principle of energy conservation, a nonlinear subgrade reaction model utilizing hyperbolic functions is developed. This model is applied to a case of a new tunnel undercrossing an existing tunnel in Shanghai, with results showing that the MSD-based calculation closely matches actual monitoring data, validating the effectiveness and applicability of the model. This paper provides a more accurate calculation method for settlement prediction and structural design in tunnel undercrossing projects, with significant theoretical and practical value.