The invention discloses a starch-modified polyvinylalcohol high-resistance degradable thin film and a preparing method. The starch-modified polyvinylalcohol high-resistance degradable thin film is prepared by smelting a polyvinylalcohol composition. The polyvinylalcohol composition is prepared from, by mass, 60-75 wt% of polyvinylalcohol, 10-20 wt% of starch modified with an oxidizing agent, 5-15 wt% of plant fibers, 1-5 wt% of natural polyphenol and 0-1 wt% of calcium oxide, wherein the mass sum of the components is 100%. By adding the starch modified with the oxidizing agent into the polyvinylalcohol base, the water resistance of the polyvinylalcohol thin film is improved while the mechanical performance of the polyvinylalcohol thin film is improved. By adding the plant fibers, the mechanical performance of the polyvinylalcohol thin film is further improved. By adding the natural polyphenol/calcium oxide composite modifying agent and the starch, the melting point of the polyvinylalcohol thin film is lowered, the melting processing window of polyvinylalcohol is enlarged, a plasticizer does not need to be added, and the reduction, caused by the plasticizer, of the mechanical performance and barrier performance of the polyvinylalcohol thin film is relieved. The prepared starch-modified polyvinylalcohol high-resistance degradable thin film is excellent in comprehensive performance, and the application range is widened.