The invention relates to a reduced graphene oxide/polyacrylonitrile composite fiber and a preparation method thereof. The method is that after graphene oxide is assembled onto the surfaces of amino group-containing polyacrylonitrile fibers by magnetic attraction and chemical bonding, reduction treatment is carried out, so that the reduced graphene oxide/polyacrylonitrile composite fiber is obtained. The average volume resistivity of the reduced graphene oxide/polyacrylonitrile composite fiber prepared by the method is less than or equal to 30 Omega.cm, reduced by 12 orders of magnitude in comparison with the average volume resistivity of the raw filament; moreover, the decrease rate of the breaking strength is less than or equal to 5 percent relative to the decrease rate of the breaking strength of the raw filament, the increase rate of the elongation at break is greater than or equal to 58 percent relative to the increase rate of the elongation at break of the raw filament, and afterdeionized water is utilized to wash the composite fiber for 10 to 20 times, the average volume resistivity of the fiber is less than or equal to 33 Omega.cm. The method disclosed by the invention hasthe advantages of reducing agent selection diversity, good reduction effect, low production cost and the like, the produced fiber has good conductivity and durability, and furthermore, the inherent excellent mechanical property of the fiber is kept.