Low-energy electronic excitations in free-standing graphene (gr) and gr(2×2)/K interface have been studied based on ab initio   band structure and linear-response theory. For pristine graphene, the calculated linear dispersion of collective interband transitions around the Dirac cone is in good agreement with experiments. At the gr/K interface, in addition to the doping-enhanced linear mode, a nonlinear plasmon develops with increasing momentum transfers along the ΓKΓK direction. Using a model-doped free-standing graphene, we revealed that the nonlinear mode originates from the anisotropic band dispersion at the Fermi level, and its collectivity emerges as the carrier density increases. These findings have implications for measurements of electronic excitations in metal-supported graphene sheet.Highlights► LR-TDLDA calculations were performed for graphene/potassium interface. ► Low-energy linear plasmon is enhanced when graphene is doped. ► A nonlinear plasmon branch is found along the ΓKΓK direction in doped graphene. ► Anisotropic band dispersion is responsible for the nonlinear mode.