Sodium ion batteries (SIBs), a promising substitute for lithium ion batteries (LIBs), have attracted extensive attention due to the abundance and low cost of sodium resources. In addition, flexible sodium-ion batteries may be able to satisfy the demands of large-scale energy storage applications for portable, wearable, and flexible electronics. Compared to the development of cathode materials, the progress on anode materials has been relatively slow. Therefore, the exploration of low-cost anode materials with high Na+ storage capacity is very important. Herein, we present oxygen-deficient Na₂Ti₃O₇ nanobelts grown on carbon cloth (CC) as a promising novel flexible anode material for SIBs. Free-standing Na₂Ti₃O₇ nanobelts with oxygen vacancies were directly grown on CC through a simple hydrothermal and thermal reduction process. Benefiting from the improved conductivity and increased active sites after the introduction of oxygen vacancies, the new material exhibits a high reversible capacity of 100 mAh.cm^-2 at 200 mA.cm^-2, with almost 80% capacitance retention after 200 cycles. When the current density was increased to 400 mA.cm^-2, a high capacity of 69.7 mAh.cm^-2 was achieved, which is three times that of bare Na₂Ti₃O₇ nanobelts on CC. This 3D oxygen-deficient electrode can significantly promote the transport of Na+ ions and electrons, leading to remarkably improved electrochemical properties. Furthermore, this work constitutes a promising strategy to rationally design and fabricate novel Na2Ti3O7-based anodes with enhanced capacitive behavior, which hold great promise for energy storage/conversion devices, facilitating the large-scale implementation of high-performance flexible SIBs.