A preparation method of an iron-coated graphene nanocomposite material comprises the following steps: (1) conducting thermal-expansion stripping on graphite oxide under a high temperature to obtain multi-layer graphene; (2) putting the multi-layer graphene and an organic solvent into a four-mouth flask, and mechanically stirring and mixing uniformly; (3) introducing pentacarbonyl iron steam into the mixed solution for thermal decomposition under a continuously stirring protection atmosphere; (4) conducting the thermal decomposition in a reflux condensation process; and (5) after the reaction,cooling to room temperature under the nitrogen protection, separating a product by a magnet, then cleaning the product by adopting an organic solvent, and drying the product under the nitrogen protection to obtain the iron-coated graphene nanocomposite material. According to the preparation method, Fe nanoparticles with a size of 20-50nm can be coated on the graphene sheet layers uniformly, the process is simple and controllable, the cost is low, and the industrial production is facilitated.