The invention discloses a master batch capable of fully and biologically degrading a plastic film and a preparation method thereof, and belongs to the field of high molecular films. The formula of the master batch comprises the following materials in parts by weight: 100 parts of starch, 2-9 parts of coupling agent, 8-18 parts of dispersing agent, 8-25 parts of polycyclolactone, 1-50 parts of polylactic acid, 8-15 parts of poly (hydroxybutyrate-hydroxyvalerate) (PHBV), 10-25 parts of plasticizer, 0.15-1.0 part of anti-tackiness agent, 0.15-1.2 parts of pro-oxidant and 1-15 parts of chemical precipitation promoter. The preparation method of the master batch comprises the three steps of: thinning the starch; mixing; and forming. The product provided by the invention has the advantages of high starch content, being a fast-growing and renewable resource, rich resource, low unit cost, high substitutability, obvious economic benefit and ecological benefit and capabilities of reducing the content of polycyclolactone, increasing polylactic acid and poly (hydroxybutyrate-hydroxyvalerate) (PHBV), improving product performance and meanwhile reducing cost.