The invention discloses an iron-based amorphous nanocrystalline composite coating and a preparation method of preparing the iron-based amorphous nanocrystalline composite coating by adopting a plasma surfacing method. Alloy powder which is prepared according to the specified proportion is subjected to surfacing on the surface of a base material by adopting a plasma surfacing machine to form an alloy coating, wherein the technical parameters comprise: the welding voltage is 25-30 V, the welding current is 140-190 A, the welding speed is 35-40 mm/min, the shielding gas flow is 10-12 L/min, the powder feed speed is 15-25 g/min and the amplitude of swing is 20-30 mm. when the plasma surfacing ends, after the coating is naturally cooled to the room temperature, the preparation of the iron-based amorphous nanocrystalline composite coating is completed. The iron-based amorphous nanocrystalline composite coating which is prepared by the plasma surfacing method is high in rigidity and good in wear-resisting property and can be widely applied to manufacturing and repair of metallurgical machinery and petrochemical engineering apparatuses. In addition, the coating prepared by the plasma surfacing method has the advantages that the technology is simple; the operation is simple and easy; no preheating before welding and no slow cooling after welding are needed; after surfacing, the coating has no cracks and no peeling, and the forming of the coating is excellent.