The invention provides a biodegradable flexible polylactic-acid alloy film, as well as a preparation method and a product thereof. The film comprises the following components: (A) polylactic-acid base resin and (B) polybasic acid polyol, wherein the weight content of the (A) polylactic-acid base resin is between 50 and 99.5 percent; the weight content of the (B) polybasic acid polyol is between 0.5 and 50 percent; and according to a ISO6721-standard test method for the dynamic mechanical properties of plastics, the storage modulus E' of the film at 25 DEG C is between 200 MPa and 4,000 MPa. On the premise of maintaining the complete biodegradation and high mechanical strength of materials, the method effectively improves the flexibility of the biodegradable flexible polylactic-acid alloy film. The film can be applied to packaging materials, agricultural film, film for labels, and the like, overcomes the disadvantages that plasticizer is easy to volatilize and exude, and maintains the transparency of the materials. The method has the advantages of effectively promoting the wide application of environment-friendly polylactic acid products, along with simple process and low cost.