The invention relates to an application of a by-product of the process of preparing sugar from starch in preparation of a polylactic acid composite material, and belongs to the field of polymer materials. The by-product of the process of preparing the sugar from the starch is obtained by the following steps: a raw amylases preparation acts on corn raw starch, the concentration of the raw starch is30%-40%, under the conditions of the temperature of 60 DEG C and the pH of 4.5-5.0, hydrolysis is carried out in one step, glucose syrup with the DE value of greater than 95% and a by-product are generated, the by-product is washed with water, and the washed by-product is dried for standby application. The application provided by the invention has the advantages that in the blending process, thesubstitution amount of the by-product to polylactic acid is greater, the usage amount of a plasticizer is smaller, the by-product of the process of preparing the sugar from the raw material in one step is effectively utilized, the costs of the polymer polylactic acid composite material are reduced, the use requirements can be met, and the degradation speed of the polylactic acid can be improved.