The flexible supercapacitors (FSCs) have attracted much attention with the development of personal electronics. Here, the polyethylene-polypropylene fibrous membranes (PE-PPFMs) are used as the substrate to prepare conductively metallized organic films (Ni/PE-PPFMs) by employing electroless nickel plating method. After graphene oxide (GO) dispersion is coated on the substrate of Ni/PE-PPFMs, electrochemical reduction is carried out to reduce GO and the electrodes of ErGO/Ni/PE-PPFMs have been obtained. The preparation conditions have been optimized and the electrodes have also been characterized in detail. The capacitive performances of the electrodes are evaluated by assembling the FSCs with the NaOH/polyvinyl alcohol (NaOH/PVA) as the gel electrolyte. The specific capacitance of the optimized electrode can reach 368.2 mF cm−2 (147.3 F g−1 based on the active materials), and the maximum energy density of the cell can reach 16.2 μWh cm−2 (or 3.2 Wh kg−1) when the power density is 0.24 mW cm−2 (48 W kg−1). Meanwhile, the cell exhibits excellent cyclic stability (the capacitance retention can still keep at 89.5% after 10,000 cycles) and flexibility (84.4% capacitance retention after 5500 bending cycles). This strategy provides a simple and environmentally friendly method to prepare flexible rGO-based electrodes.