Graphene quantum dot-decorated porous carbon spheres were synthesized via a facile and green route in this study. The presence of graphene quantum dots modified the conductivity of the resultant materials. Moreover, the obtained self-doped heteroatom carbon material demonstrated a high specific surface area of up to 1393.9 m² g⁻¹. Based on the above merits, the obtained composites reveal superior electrochemical performance for energy storage. The specific capacitance of the composite-based single electrode and assembled flexible solid-state supercapacitors was found to be 237.3 F g⁻¹ and 244.5 mF cm⁻², respectively. Meanwhile, the device's energy density can reach 21.7 μW h cm⁻² at a power density of 674.5 μW cm⁻². Impressively, despite the device's power density of 8000 μW cm⁻², the energy density still remains at 18.8 μW h cm⁻². In addition, the assembled device delivers outstanding flexibility. Even after the device is bent at 154.4°, the energy density maintains a value of 13.7 μW h cm⁻². In brief, a feasible reference for the preparation of functional carbon materials for high-performance energy storage devices was presented via a high efficiency, low cost, and green route.