The invention relates to a melt /
solid state polycondensation preparation method for
polylactic acid, which mainly solves the problems of long reaction time, difficult
continuous production or high cost in the prior art. The method comprises the following steps of: 1) performing a
dehydration oligomerization reaction of
lactic acid at the temperature of between 100 and 160 DEG C under the absolute pressure of between 6,000 and 25,000 Pa and under the action of a catalyst A for 0.5 to 5 hours first, and continuing the ehydration oligomerization reaction at the temperature of between of 120 and 180 DEG C under the absolute pressure of between 200 and 1,500 Pa and under the action of the catalyst A for 0.5 to 5 hours to generate a product I; 2) performing a melt
polycondensation reaction of the product I at the
reaction temperature of between 150 and 200 DEG C under the absolute pressure of between 30 and 600 Pa and under the action of a catalyst B for 0.5 to 15 hours to generate a
polylactic acid prepolymer and
lactide serving as a by-product, and refluxing the
lactide to a reaction mixture at the
reflux temperature of between 70 and 95 DEG C; 3)
pelletizing the
polylactic acid prepolymer obtained in the step 2), and crystallizing in an
inertia airflow at the temperature of between 50 and 130 DEG C for 0.5 to 10 hours; and 4) performing a
solid state
polycondensation reaction of the polylactic acid
prepolymer processed in the step 3) in the
inertia airflow at the temperature of between 120 and 170 DEG C for 5 to 40 hours to obtain the polylactic acid. The technical scheme of the invention better solves the problem, and can be used for the industrial production of the polylactic acid.