The present disclosure relates to a polymer-based large-area carbon nanomesh and a method for preparing same. More particularly, the present disclosure provides a method for preparing a carbon nanomesh, including: preparing a polymer nanofilm by coating a solution of a block copolymer or a polymer mixture thereof on a substrate; stabilizing the polymer nanofilm by annealing such that the polymer nanofilm is phase-separated, a pore-forming polymer is removed and, at the same time, a nanomesh-forming polymer forms a stabilized porous polymer nanomesh; and carbonizing the stabilized porous polymer nanomesh by annealing at high temperature to prepare a carbon nanomesh. Using phase separation and cyclization of a polymer, a large-area carbon nanomesh with superior activity can be prepared simply with high reproducibility in large scale.