The invention discloses a preparing method of a graphene steel composite. The preparing method comprises the steps that mixed liquor is provided and comprises an organic solvent, graphene powder and steel powder; the mixed liquor is subjected to drying treatment to obtain mixed powder; the dried mixed powder is loaded in a sheath to be subjected to vacuum pumping degassing treatment, and the sheath is sealed; the sealed sheath loaded with the mixed powder is subjected to hot isostatic pressure treatment, the temperature of the hot isostatic pressure ranges from 920 to 1300 DEG C, and the mixedpowder forms a composite green body; and the composite green body and the sheath are separated.