A 400-mu m-thick free-standing and flexible cellulose/RGO/AgNPs composite film was fabricated by chemical reduction of cellulose/GO/AgNPs film with hot hydrazine vapor, which was obtained via the reaction of GO with silver-ammonia complex in the presence of cellulose fibers, followed by dewatering with assistance of filtration. The 400-pm-thick cellulose/RGO/AgNPs film was systematically characterized and found to be conducive enough with sheet resistance of merely 0.17 Omega sq(-1) for direct use as electrode. When measured in a three-electrode setup, such flexible electrode exhibited outstanding supercapacitive performances with the highest areal specific capacitance of 1242.7 mF cm(-2) (corresponding to 31.1 F cm(-3)), excellent rate capability and remarkable cycling stability with the capacitance retention of 99.6% after extensive charge/discharge at a high current density of 60 mA cm(-2) for more than 10,000 cycles. Furthermore, a symmetric flexible all-solid-state supercapacitor cell of cellulose/RGO/AgNPs//cellulose/RGO/AgNPs was assembled. It delivered the maximum areal specific capacitance of 683.8 mF cm(-2) (corresponding to 8.6 F cm(-3)), possessed desirable rate capability, showed no substantial variation of supercapacitive behavior when being bent, and offered the highest areal energy density of 95.0 mu Wh cm(-2) (corresponding to 1.19 mWh cm(-3)), which outperformed that of many existing graphene-based symmetric flexible supercapacitors and lots of asymmetric flexible supercapacitors. The impressive electrochemical properties and facile preparation process of cellulose/RGO/AgNPs film make it a promising candidate for constructing next-generation high-performance flexible energy storage devices.