The invention relates to a surface modification method of carbon fibers. Graphene oxide is used as a surface modifying material of the carbon fibers; and the graphene oxide is deposited on the carbon fibers through ultrasonic-assisted electrophoresis and can form a composite material with high performance. The surface modification method is low in cost, simple to operate and strong in applicability. The surface modification method of the carbon fibers has the advantages that equipment for surface modification of the carbon fiber is low in investment cost, and the method is low in cost, simple in operation, strong in applicability, good in treatment effect, small in fiber performance loss and reliable in quality. The modification time is greatly shortened, the use amount of chemicals and the cost of the product are reduced, and the environmental pollution is reduced, so that the surface modification method is suitable for industrial production.