The invention provides a degradable mulch film and a preparation method thereof, which belongs to the technical field of high polymer materials. The degradable mulch film comprises, by weight, 10 to 20 parts of polyethylene, 10 to 20 parts of polypropylene, 2 to 3 parts of polyvinyl alcohol, 1 to 2 parts of poly propylene carbonate, 3 to 5 parts of polylactic acid, 1 to 2 parts of poly(3-hydroxybutyrate-co-4-hydroxybutyrate), 4 to 6 parts of chitosan, 2 to 4 parts of methyl-polyolefin resin, 2 to 4 parts of polypropylene glycol, 0.5 part of nanometer titanium dioxide, 0.5 part of a photolysis agent, 5 to 15 parts of an inorganic filling material and 5 to 15 parts of starch. The degradable mulch film provided by the invention has the following advantages: 1, a fast degradation fast, and a fast degradation rate of more than 35% within 120 d; 2, high strength, transverse tensile strength of more than 16 MPa and vertical tensile strength of more than 17 MPa.