To meet the rapid development of lightweight, flexible, and even wearable electronics, it is critically important to develop matchable, highly efficient energy-storage devices for their energy supply. Graphene fiber-based supercapacitors (SCs) are considered as one of the promising candidates because of the superior mechanical and electrical properties of graphene fibers. However, SCs based on neat graphene fibers generally suffer a low capacitance and poor rate performance, which largely restrict their potentially wide applications. Here, we report a simple, low cost and scalable wet-spinning method to fabricate porous carbon black/reduced graphene oxide (CB/rGO) hybrid fibers. The hybrid fibers possess very high surface area (254.6 m² g⁻¹) and a hierarchically porous nanostructure. A flexible solid-state SC was assembled using the hybrid fiber, which exhibited high capacitance (97.5 F cm⁻³), excellent cycling stability (95.9% capacitance retention over 2000 cycles), superior energy density (2.8 mW h cm⁻³) and ultrahigh power density (1200 mW cm⁻³). Its physical shape and electrochemical performance is also very well maintained under long-time periodic mechanical deformation that is particularly promising for wearable electronic devices.