Confining light in extremely small cavities is crucial in nanophotonics, central to many applications. Employing a unique nanoparticle-on-mirror plasmonic structure and using graphene film as spacer, we create nanoscale cavities with volumes of only a few tens of cubic nanometers. The ultracompact cavity produces extremely strong optical near fields, which facilitate the formation of single carbon quantum dot in the cavity and simultaneously empowers the strong coupling between the excitons of the formed carbon quantum dot and the localized surface plasmons. This is manifested in the optical scattering spectra, showing magnificent Rabi splitting up to 200 meV at ambient conditions. In addition, we demonstrate that the strong coupling is tuneable with light irradiation. This opens new paradigms of investigating the fundamental light emission properties of carbon quantum dots in the quantum regime and paves the way for many significant applications.