The invention provides a graphene / polysiloxane composite coating material and a preparation method thereof. The method comprises the following steps: modifying the graphene surface with active groups by a chemical modification technique to obtain modified graphene, mixing the modified graphene with a silane compound, a dispersion medium, a non-essential comonomer and a non-essential accelerator, and carrying out hydrolytic condensation or hydrolytic condensation-free radical polymerization to generate a graphene / polysiloxane composite resin in situ; and mixing the resin with a non-essential blend resin, a non-essential curing agent, a non-essential solvent, a non-essential pigment and filler and a non-essential aid, and carrying out physical blending, amino epoxy addition reaction, Michael addition reaction and the like to form a film, thereby obtaining the graphene / polysiloxane composite coating material. The uniformly dispersed graphene lamellae and polysiloxane have strong interface effects, also have the gas / liquid barrier and heat shielding effects, and endow the coating with excellent corrosion resistance, flame retardancy and the like, thereby greatly enhancing the mechanical properties, corrosion resistance, scratch resistance, wear resistance and the like of the coating.