Two-dimensional (2D) nanomaterials hold considerable potential in reforming the energy storage performance, and the efficient production of high-performance 2D energy storage materials through facile and sustainable approaches is highly desirable. Herein, for the first time, large-area and ultrathin carbon nanosheets doped with N/O were constructed by stripping bulk chitin via a "top-down" method. On the basis of the specific layered structure composed of nanofibers, chitin samples after removing the protein and CaCO3 could be efficiently exfoliated into nanosheets (CNs) via the hydrothermal method, which were then carbonized into N/O co-doped porous carbon nanosheets (CCNs). The CCNs with a thickness of about 3.8 nm retained the original nanosheet structure consisting of nanofibers, leading to a 2D structure with hierarchical porosities. When used as anode materials for sodium-ion batteries, the 2D porous nanostructures and abundant N/O doping of CCNs-600 (carbonized at 600 °C) enable a high reversible capacity of 360 mA h g-1 at 50 mA g-1, a good rate capability of 102 mA h g-1 at 10 A g-1, and an excellent cycling stability of 140 mA h g-1 after 10 000 cycles at a high density of 5 A g-1. Full cells consisting of a CCN anode and a Na3V2(PO4)3/C cathode exhibited favorable rate performance and cycling stability, showing potential application prospects in highly efficient energy storage systems.