Graphene fiber-based supercapacitors are emerging as one of the most promising energy-storage devices for wearable electronic devices. However, neat graphene fibers fabricated from liquid crystal spinning, the most commonly used method for preparing graphene fibers, usually have a graphite-like structure due to the π–π aggregation, which greatly impedes their practical application in energy storage. Herein, we developed a facile approach for fabricating a N and S co-doped porous graphene fiber by means of microgel spinning using self-assembled 3D GO microgels as a spinning dope followed by thermal reduction. The fabricated fiber with a large specific surface area (312 m² g⁻¹), appropriate hierarchical pore structures and synergistic effects of N and S co-doping can act as a superior flexible electrode for fiber-shaped supercapacitors in terms of high volumetric capacitance (59.9 F cm⁻³ at a current density of 0.1 A cm⁻³), outstanding energy and power density (8.3 mW h cm⁻³ at 50.3 mW cm⁻³), exceptional rate capability (44.1 F cm⁻³ at a high current density of 1 A cm⁻³) and long cycle stability (96.2% of the initial specific capacitance retention over 10 000 cycles). This cost-effective and simple preparation method paves the way to large-scale fabrication of heteroatom-doped porous graphene fibers and promotes their practical application in wearable devices.