The invention belongs to the technical field of polymer chemistry, relating to a preparation method of biological medical degradable material polylactic acid. A method for synthesizing the polylactic acid by using a catalytic one-step method and taking calcined kaoline as a catalyst is characterized by comprising the following steps: (1) lactic acid dehydration: dehydrating the taken lactic acid for 1-5 hours in the presence of inert gas, wherein the dehydration temperature is 70 DEG C-150 DEG C, and the vacuum is 10-200 mmHg; and (2) direct melt polycondensation: carrying out a melt polycondensation reaction on the dehydrated lactic acid by taking the calcined kaoline as the catalyst under anhydrous anaerobic conditions, so as to obtain a polymer, and then carrying out post-treatment on the polymer so as to obtain the polylactic acid product. In the method, cheap and available calcined kaoline is used as a catalyst, the reaction yield is high, and the biology safety of the polylactic acid obtained by the one-step method is high.