The invention discloses graphene carbon fiber and a preparation method thereof. The preparation method includes: (1) mixing dimethylformamide dispersion or dimethyl sulfoxide dispersion containing graphene oxide with polyacrylonitrile powder and a bridging agent, storing at room temperature with air isolated, allowing swelling, heating under stirring to carry out dissolving to obtain polyacrylonitrile-graphene oxide spinning solution; (2) extruding the polyacrylonitrile-graphene oxide spinning solution out of a spinneret in metered manner, forming fiber in a solution containing at least one ofethanol, water, dimethylformamide and dimethyl sulfoxide, drafting, washing with water, oiling, drying, and winding to obtain polyacrylonitrile-graphene oxide raw fiber; (3) peroxidizing the polyacrylonitrile-graphene oxide raw fiber, and carbonizing to obtain graphene carbon fiber. The preparation method herein is suitable for preparing graphene fiber with high-dispersity graphene oxide solutionhaving fiber forming difficulty; the graphene carbon fiber has graphene content of not less than 75% and heat conductivity up to 1520 Wm<-1>K<-1>.