The urban track traffic develops rapidly with the economic growth in China. Among the common diseases of shield tunnel, longitudinal settlement will put adverse influence on the longitudinal performance of shield tunnel. The structural stiffness of shield tunnel structure is one of the most important mechanical performance index. The longitudinal equivalent continuous model is adopted; and the longitudinal effective rigidity ratio is assumed to be constant and variable longitudinally. The longitudinal equivalent rigidity is back calculated based on settlement curve to analyze the influence of longitudinal settlement on the longitudinal equivalent rigidity of shield tunnel. The results indicate: (1) When the longitudinal equivalent rigidity is constant, the longitudinal effective rigidity ratio reduces and change to be steady as the ratio of actual settlement and theoretic settlement increases. (2) When the longitudinal effective rigidity ratio is variable longitudinally, the back-calculated at the middle part is larger than that at both ends less than 1; and the back-calculated at the middle part is smaller than that at both ends for are greater than1. (3) The settlement curve which is calculated based on back-calculated has some differences with the actual settlement curve.