The fast development of portable devices has greatly stimulated the demand for flexible supercapacitors (FSCs). The key to assembling FSCs lies in the development of electrodes with high-performance and good flexibility. In this work, oxygeneous groups modified nitrogen-doped carbon fibers cloth (ONCC) was obtained through annealing the pre-oxidized polyacrylonitrile cloth (PPC), followed by a facile oxidation process using a mixture of concentrated sulfuric acid and concentrated nitric acid. By changing annealing temperature, the nitrogen content can be controlled in the obtained ONCC. It was found that the PPC annealed at 950 degrees C followed by acid etching for 60 min could deliver the excellent areal and gravimetric specific capacitance of 1385 mF cm(-2) and 294.7 F g(-1) at the current density of 1 mA cm(-2), outstanding rate performance and superb cycling stability. Furthermore, the assembled FSC delivered a high energy density of 19.8 mu Wh cm(-2) (4.03 Wh kg(-1)) at a power density of 50.1 mu W cm(-2) (10.2 W kg(-1)) and good flexibility under different bending conditions. This work provides a facile and efficient route to prepare high-performance carbon fiber based electrode materials in large scale for flexible all-solid-state energy storage devices.