The invention provides composite petroleum coke-based activated carbon and a preparation method thereof. Activated active sites can be formed on the surface of petroleum coke through the dipping, coking and pre-activating processes of an oxidizing medium, chemical activation part pore-forming is promoted, the use amount of alkali is reduced, the mesopore rate is increased, and the problem that due to the fact that the surface of the petroleum coke is compact, pore forming is difficult is solved. Graphene oxide is reduced into graphene, a conductive network is formed, the conductivity of the activated carbon is improved, and the internal resistance of the activated carbon is reduced. High-temperature heat treatment is carried out jointly, surface functional groups are reduced, and the stability of super-capacitor carbon is improved. The prepared composite petroleum coke-based activated carbon has the advantages of being reasonable in pore size distribution, large in specific surface area, low in internal resistance, high in capacity, low in impurity and low in oxygen-containing functional group and can serve as the super-capacitor carbon, and the super-capacitor with better performance is obtained.