The invention discloses a mesophase-pitch-based carbon fiber and a preparation method thereof. The preparation method comprises the steps of conducting mesophase pitch polymerization, modification and visbreaking, conducting centrifugal spinning, conducting hot jetting drafting, conducting oxidation, carbonization and graphitization, and obtaining the mesophase-pitch-based carbon fiber. The diameter of the mesophase-pitch-based carbon fiber ranges from 120 nm to 2000 nm, the draw ratio is 500-20000: 1, the tensile strength is 1.5-4.0 GPa, the tensile modulus is 300-600 GPa, the electrical resistivity is less than 0.2 * 10 <-4> omega.cm, the thermal conductivity is 500-800 W/m.k, and the mesophase-pitch-based carbon fiber is low in fabrication cost and can be applied to fields such as static electricity resistance, heat dissipation, electromagnetic shielding and heat shielding.