The development and utilization of renewable clean energy has become a common way out for the world to solve energy crisis. The concentration gradient between sea water and river water is widely regarded as a very significant sustainable energy resource because of its easy availability and abundant reserves. Therefore, it is necessary for rapid technical breakthrough of membrane engineering in order to capture this energy existing in the fluidic system. Herein, we develop nanofluidic device that can harvest osmotic energy and rectify ionic transport by directly prepared with a nanoporous TOCNs membrane and a conical variable-channel porous polyethylene terephthalate (PET) substrate membrane. 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) oxidized cellulose nanofibers (TOCNs) nanofluidic device possesses the advantages of well-tunable geometry and high charges density, which develop an attractive material for the control of ion flow. The optimized TOCNs heterogeneous membrane shows prominent cation selectivity and ion current rectification ratio of 562. When applying this TOCNs heterogeneous membrane for a concentration-gradient-driven device, a high power density reaches 0.96 W/m2, which exhibits great promise for energy harvesting device. Overall, this work provides an effective way for devising cellulose nanofibers-based nanofluidic device and can promote the development of concentration-gradient-driven energy conversion system.