The invention provides a method for preparing a multifunctional nano protective coating by circulating large-duty-ratio pulse discharging and belongs to the technical field of plasma. The method is characterized in that a reaction chamber is vacuumized, inert gas is filled into the reaction chamber, a substrate can move, monomer steam is filled into the reaction chamber, and chemical vapor deposition is carried out; a deposition process comprises a pretreatment stage and a film coating stage; the plasma discharging manner in the pretreatment stage is a high-power continuous discharging manner; the plasma discharging manner in the film coating stage is a large-duty-ratio pulse discharging manner; the pretreatment stage and the film coating stage are circulated and repeated for at least one time; through circulation introduction in the film coating process, more active sites are introduced into the surface of the substrate; the effective coating of the film can be increased; the film structure is more dense; a nano coating with a multilayer composite structure is obtained; multi-level protection is provided for the product; the product shows more dense coating structure at a micro level and shows excellent hydrophobicity, adhesion, acid resistance, alkali resistance, mechanical property, moisture resistance and heat resistance at a macro level.