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In the existing polylactic acid production, lactide is firstly prepared from lactic acid or purchased, and then lactide is subjected to ring-opening polymerization by using a relatively expensive catalyst such as metal zinc elementary substance, so that polylactic acid is prepared in two steps. According to the scheme, food-grade zinc acetate dihydrate is used as a catalyst for catalyzing lactic acid to accelerate dehydration condensation reaction, and lactic acid obtained by fermenting lactic acid bacteria after saccharification of starch is subjected to one-step catalysis is heated, stirred and polycondensed at a proper temperature to prepare polylactic acid. The whole process is free of disordered interference reaction, and all reactant products belong to food grade, are non-toxic and harmless, and are ecological and environment-friendly. Complex and expensive instruments, equipment and reagents are not needed. Zinc acetate dihydrate is easily dissolved in a lactic acid aqueous solution, ionized zinc ions have two valence states of + 1 valence and + 2 valence, an empty electron orbital and a unique Ar 3d10 electronic structure and have strong electronegativity, the + 1 valence zinc ions lose or obtain one electron, the surfaces of the zinc ions have rich catalytic active sites, and the catalytic activity of the zinc ions is improved. In an acid environment of a lactic acid solution, zinc ions catalyze hydroxyl carboxyl contained in lactic acid molecules to accelerate transition loss of electrons with different probabilities and probabilities or shift deflection of direction, angle and the like of shared electrons, so that water molecules are formed and removed, and dehydration condensation of hydroxyl carboxyl in lactic acid molecules and between lactic acid molecules is continuously catalyzed; and carrying out polycondensation on the oligomeric lactic acid to obtain polylactic acid. Meanwhile, a small amount of intermediate lactide formed by heating and catalyzing is also subjected to ring-opening polymerization reaction to form a linear polylactic acid chain.