A kind of compound of alkali metal potassium and carbon C4K with Pmmm symmetry was designed by inserting alkali metal potassium between two-dimensional carbon material M-graphene. The crystal structure, thermodynamic stability and electronic properties of C4K are systematically studied via the first-principles calculation of density functional theory. Results show that C4K is thermodynamically stable at high pressure, it can be synthesized in 7.5 - 24.5 GPa, and presents electronic properties of metal. This work provides some theoretical guidance for experimental synthesis of alkali metal and carbon compounds.