A flexible binder-free film composed of bismuth oxide nanoparticles embedded in carbon nanofibers (Bi2O3/C) was prepared by a feasible electrospinning method and directly used as a sodium ion battery (SIB) anode. As a binder-free and flexible anode for SIBs, Bi2O3/C delivers a high reversible capacity of 430 mA h g(-1) after 200 cycles at a current density of 100 mA g(-1) and an exceptional rate capability of 230 mA h g(-1) at 3200 mA g(-1). It has a stable capacity of 252 mA h g(-1) after 50 cycles at 400 mA g(-1) in a Na-ion full cell device. The high capacity, good cyclability and rate capability are attributed to synergistic effects of the uniform distribution of ultra-small Bi2O3 nanoparticles (approximate to 10 nm) in the carbon nanofibers and the conducting framework of 3-D interconnected carbon nanofibers, which can effectively alleviate the volume expansion during sodiation/desodiation processes and maintain the high electrical conductivity throughout the electrode. This self-standing flexible Bi2O3/C nanocomposite electrode may hold great promise for high-performance SIBs.