Abstract: This paper focuses on solving the problem of geometrically nonlinear buckling of thin-walled cylin- drical shells under pure bending with the cross-section tending to be oval-shaped. The basic hypothesis is based on the modified Brazier simple theory that the deformation of a shell under pure bending can be simplified as a two-stage process. By the two-stage process, the longitudinal bending strain energy and the cross-sectional deformation strain energy are obtained. The relationship between the end-rotation and the applied moment is obtained through the principle of the minimum potential energy. It is shown that: the smaller the shell length parameters and the thinner the corresponding cylindrical shell wall, the greater the impact of nonlinearity will be; the smaller the shear length parameters and the smaller the effect of the boundary conditions on the deformation leading to oval section shape, the greater the impact of nonlinearity will be.