Sodium-ion batteries (SIBs) are an attractive battery system because of similar characteristics to lithium-ion batteries (LIBs) and large Na element abundance. Nevertheless, exploring stable, high-capacity and high-rate anode materials for SIBs is still challenging now. Herein, diethylenetriamine (DETA) molecular template derived ultrathin N-doped carbon (NC) layer decorated CoSe₂ nanobelts (CoSe₂/NC) are prepared by solvothermal reaction followed by calcination process. The CoSe₂/NC exhibits large potential as an anode for SIBs. Experiments and theoretical calculations reveal that the in situ formed conductive ultrathin NC layer can not only relieve the volume change of CoSe₂ but also accelerate electron and ion transport. In addition, the nanobelt structure of CoSe₂/NC with abundant exposed active sites can obviously accelerate the electrochemical kinetics. Under the synergistic effect of special nanobelt structure and NC layer, the rate as well as cycling performances of CoSe₂/NC are obviously improved. A superior capacity retention of 94.8% is achieved at 2 A·g⁻¹ after 2000 cycles. When using Na₃V₂(PO₄)₃ cathodes, the pouch full batteries can work steadily at 0.5 C, verifying the application ability. CoSe₂/NC anodes also exhibit impressive performances in LIBs and potassium-ion batteries (PIBs).