With development of flexible electronics, investigation on flexible supercapacitors is becoming more and more important. How to easily prepare flexible electrodes with high specific capacitance is still challenging. Here, Ni3S2-based flexible positive electrode is obtained by hydrothermally treating the continuous Ni film which has been chemically deposited on flexible organic fibrous substrates with thioacetamide. The structural character, electrochemical properties and the influences of reduced graphene oxide on the microstructure and properties of the electrode are investigated in detail. Results reveal that the formed Ni3S2 shows a neural network-like structure. The presence of reduced graphene oxide makes the Ni3S2 size small, improves the dispersibility of Ni3S2 and increases conductivity of the electrode. The specific capacitance of the optimized electrode can reach to 2.3 F cm−2 (990.1 F g−1 based on the mass of reduced graphene oxide and Ni3S2). In addition to the good flexibility, the asymmetric supercapacitors assembled by using Ni3S2-based positive electrode and reduced graphene oxide negative electrode exhibit high capacitance (680.5 mF cm−2 or 81.9 F g−1), excellent cycle stability (retaining 86.1% of the initial capacitance after 5000 cycles) and high energy density (29.1 Wh kg−1). This strategy may provide new approach for nickel-based flexible electrodes.