The invention provides a polylactic resin composition film and a preparation method thereof, and relates to the field of plastic processing. The polylactic resin composition film comprises the following components: 100 weight parts of polylactic resin and 0.5 to 5.5 weight parts of functional filler which are blended, wherein the polylactic resin has the weight-average molecular weight of 80,000-300,000 and the molecular weight dispersibility of 1.5-2.2; and the functional filler is at least one of the following inorganic fillers: calcium carbonate, clay, silica, alumina, barium sulfate, calcium sulfate, talc, magnesium sulfate, titanium dioxide, zeolite, aluminum sulfate, kieselguhr, kaolin, calcium oxide and magnesium oxide. The preparation method comprises the following steps of: melting, blending and granulating the components in the proportion, casting and pressing, and biaxially tensioning to obtain the polylactic resin composition film. The film is completely biodegradable, has balanced rigidity and toughness, maintains the tensile strength, has the elongation at break improved by 3 to 7 times compared with the original elongation at break, and has uniform texture, high decorative property, high printing property and unidirectional tearing property.