The invention provides a preparation method of a graphene-activated carbon composite material. The preparation method comprises the following steps: immersing activated carbon in a mixed solution of graphene and hydrogen peroxide to obtain activated carbon; immersing the activated carbon in an immersing agent containing ammonia and copper hydroxide; drying to obtain the graphene-activated carbon composite material. Compared with the prior art, the invention has beneficial effects that the graphene-compounded activated carbon of the invention has a protection time 1.5-2 times that of common activated carbon on the market at present, and has advantages of being simple in process, environmentally friendly and free of pollution.