Hard carbon materials are considered as one of the most promising anodes for sodium-ion batteries (SIBs). Here, the carbon nanosheet frameworks (CNFs) derived from sodium alginate without activation or any further treatments. CNFs have large cavities, thin walls and large interlayer distance (0.39nm), resulting in a high reversible sodium storage capacity (216mAhg−1 at 100mAg−1) and a good cycle stability (160mAhg−1 at 100mAg−1 over 340 cycles) in SIBs. Meanwhile, CNFs show excellent rate capability (66mAhg−1 at 5000mAg−1). The high capacity, excellent cycle performance combined with facile synthesis procedure make CNFs to be a promising anode material for practical SIBs.