Abstract Longitudinal differential settlement of shield-driven tunnel may cause longitudinal bending of the structure, which may further introduce flattening of the tunnel. The flattening of a tunnel as well as the additional internal forces and deformations it causes is seldom studied in the literature. In this paper, analytical solutions are first developed for estimating the additional forces and deformations caused by flattening of a jointed shield-driven tunnel. The effects of tunnel joints, tunnel diameter, lining thickness, ground resistance and original tunnel curvature on the flattening effect of the tunnel are discussed. An application example of a tunnel in Shanghai is adopted to illustrate the flattening of the tunnel. It is found that the flattening effect is significant if the bending radius of tunnel is smaller than 5000m, and that the deformations and internal forces induced by the flattening effect is comparable with those induce by the earth pressure. Thus, if the flattening effect is ignored, the design may be unsafe. It is important to consider the additional internal forces due to tunnel flattening in tunnel design when differential settlements are expected.