<p>This paper proposes a new theoretical approach to analysis the mechanism of the post buckling and the initiation of a propagating buckle in subsea sandwich pipelines based on shear deformation theory. Assuming that the buckling extends along the longitudinal axis of the sandwich pipelines over a certain length, which varies at different post-buckling stage, governing equations for post-buckling of the sandwich pipelines subject to external pressure were established using von Karman’s geometric nonlinear relationship. A plausible function is selected to depict the transverse deformation of the sandwich pipeline, from which the profile of the propagating buckle is determined. The total energy stored in sandwich pipelines with soft core is formulated and a set of nonlinear algebraic equations describing the post buckling equilibrium state is built from the principle of minimum potential energy. The accuracy of the analytical solutions are justified against the finite element results and the experimental results from the existing literatures. It is found that once buckling occurs, the cross section of the pipeline is crushed down into a dog-bone shape. In addition, the length of the post buckling region is not related to the material properties of the sandwich pipeline, but only to its geometric parameters.</p>