Organic cathode materials are of great interest for application in batteries due to their abundant availability and environmental compatibility. An approach to make long chain molecules of these organic materials in order to overcome the problem of dissolution in a liquid electrolyte (LE) and incorporate a highly conducting material to enhance the poor electric conductivity of these materials would be of great research interest. In this work, a novel polyimide (PI)/multiwalled carbon nanotube (MWCNT) nanocomposite is prepared as the cathode material for organic Na-ion batteries (NIBs), via a two-step imidization reaction using perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and diaminopropane (DAP) to form an insoluble PI. The MWCNT in the composite serves as the conductive channel to maximize the utilization of the active material in the electrode. Furthermore, a three-dimensional fiber network is prepared from electrospun polyacrylonitrile nanofibrous membrane and used as a gel polymer electrolyte (GPE) with efficient electrolyte uptake and high ionic conductivity. The combination of the PI/MWCNT nanocomposite cathode and GPE results in a highly efficient organic NIB with an ultralong life span of 3000 cycles and stable cycle performance at high C-rates.