This paper employs an atomic-scale finite element method (AFEM) to study the postbuckling behavior of carbon nanotubes (CNTs). The computed energy curves and critical strain for the (8, 0) single-walled CNT (SWNT) agree well with atomistic simulations. The AFEM is very fast and versatile owing to the efficiency of the finite element method. For the SWNT, the strain energy curves have obvious jumps at morphology changes, and during the smooth continuation stages of postbuckling, the strain energy varies approximately linearly with the strain. For the double-walled CNT, there are only small strain energy releases, and the strain energy also changes approximately piecewise linearly with the strain. The morphologies are obtained in detail. AFEM is computationally fast and is an alternative efficient way to study the postbuckling of CNTs.