Na+ pre-intercalated bilayered vanadium oxide (NVOnH) with a large interlayer spacing of ∼11.1 Å is synthesized via hydrothermal method. As proven by mass spectrometry and magnetization measurements, NVOnH is “oxygen-rich”. NVOnH undergoes a notable structure evolution and thermal instability during heating due to crystal water and O2 release and decomposition. NVOnH and dehydrated NVOnH (NVO) deliver a capacity of over 250 mAh g−1 and 220 mAh g−1, respectively, despite their fast capacity decay in the first 20 cycles and low capacity retention after 100 cycles. Moreover, the reaction mechanism and reversibility of NVOnH and NVO during Na+ ions (de)insertion are investigated via in operando techniques. NVOnH experiences a 2-phase and solid solution reaction during discharge and charge processes while NVO undergoes a different phase evolution (NVO starting with charging: 2-phase reaction for the first charging and only solid solution for the following cycle; NVO starting with discharging: only solid solution reaction). In operando X‑ray absorption spectroscopy demonstrates the variation of the oxidation state and the local structural environment of V ion during Na+ (de)insertion.