Na-containing bilayer vanadium oxides offer outstanding storage capabilities for lithium and sodium batteries, but a multistep process for chemical pre-intercalation of Na-ion exposes a limitation for the application. Herein, we propose a facile one-pot hydrothermal approach to synthesize well crystallized bilayer Na x V2O5·nH2O with ketjen black as conductive dense-network. As a cathode in sodium-ion batteries, the resultant NaxV2O5·nH2O/ketjen black nanocomposite exhibits a high specific capacity of 239 mA h g-1 at a current density of 20 mA g−1 as well as an enhanced rate performance at high current rates up to 640 mA g−1. Ex situ XRD and XPS analysis are conducted to demonstrate the high specific capacity and investigate the changes in structure. Moreover, it is noticeable that two pronounced voltage plateaus reflect the high electrochemical activity of V4+/V5+, contributing to a remarkable energy density of 597 Wh kg−1, which is one of the highest records for cathode of sodium-ion batteries ever reported.