As an emerging energy storage material, amorphous Co–Ni–B alloy was firstly introduced to construct the flexible supercapacitor electrode. To ensure the high electrochemical property, amorphous Co–Ni–B alloy and RGO sheets were combined to form the three-dimensional hierarchical structure on the surface of the cotton fabric, which was beneficial to enhance the electrochemical property. Notably, the preparation conditions of this amorphous Co–Ni–B/RGO/fabric electrode were facile and mild with room temperature and atmospheric pressure, thus avoiding serious damage to the textile fabric because of high temperature and harsh chemical reactions of most preparation methods. This flexible electrode exhibited an optimum specific capacitance of 313.9 F g⁻¹ at 5 mV s⁻¹ and good cycling stability with specific capacitance retention of 85.0% after 3000 cycles. Such special architecture bestowed this electrode with nice electrochemical property, in addition to great promising application in the supercapacitor field.