The invention relates to a method for separating and refining polymer grade L-lactic acid from a zymotic fluid containing lactic acid through a dual-membrane process. The method comprises the following steps that: the zymotic liquid is subject to ceramic membrane filtration, discoloring, acidification, discoloring, crude product condensation, discoloring, ion exchange, nano-filtration, condensation and other processes to generate the polymer grade L-lactic acid; the zymotic liquid directly pass through a ceramic membrane and is filtered for removing thalli, macromolecular protein and other main impurities in the zymotic liquid; a 302# active carbon is adopted to discolor the zymotic liquid and remove micromolecular substances of a pigment class; an ion exchange process is adopted to effectively remove various ions, micromolecular proteins and other substances in material liquid; and a nanofilter membrane is adopted to filter the material liquid after ion exchange and further remove micro ions and the micromolecular substances, thereby generating a high-purity and high-quality L-lactic acid through concentration. The process method has small equipment investment and low production cost, is widely suitable for preparing the high-purity and high-quality polymer grade L-lactic acid and has good application prospect.