The invention discloses a preparation method of a polylactic acid modifier, which comprises the following steps: adding previously dried polylactic resin powder in a sodium hydroxide or potassium hydroxide solution with the concentration of 0.5-10mol/L, reacting at 30-90 DEG C for 0.5 hour to 10 days, neutralizing with hydrochloric acid until the pH value is 4.0+/-0.1, repeatedly washing and filtering with distilled water and then removing filtrate, and finally drying the obtained residual solid to constant weight. The obtained powder is the novel polylactic acid modifier with higher relativemolecular mass. The invention also discloses application of the polylactic acid modifier in the preparation of a polylactic acid film. In the invention, the low-relative-molecular-mass part in the polylactic resin can be removed based on the water solution reaction; and therefore, the polylactic acid product with higher relative molecular mass and concentrated distribution can be obtained, and the polylactic resin is modified. The method has the advantages of wide raw material source, good market prospect, simple preparation process and low requirement for equipment, is easy to control, can realize industrialization, and saves investment.