The disclosure discloses a method for producing α-calcium sulfate hemihydrate whiskers by using fermentation broth for producing lactic acid with a calcium salt method as a raw material and synchronously recovering a lactic acid monomer. The method comprises the following steps: 1) after fermentation of lactic acid is ended, heating fermentation broth; 2) stirring, and adding sulfuric acid for reaction; 3) after the reaction is ended, filtering and collecting a solid part, namely α-calcium sulfite hemihydrate whiskers, and collecting a liquid part, namely a free lactic acid solution containing the lactic acid monomer; and 4) washing and drying the obtained α-calcium sulfate hemihydrate whiskers to obtain a α-calcium sulfate hemihydrate whisker finished product, filtering and concentrating the obtained free lactic acid solution to obtain a lactic acid crude product, and refining the lactic acid crude product to obtain a high-purity lactic acid monomer. The disclosure can replace the efficient separation of lactic acid in production of lactic acid with the existing calcium salt method and high value-added transformation of a calcium sulfite byproduct, thereby significantly reducing the refining cost of lactic acid and formation of wastes and facilitating improvement of lactic acid production quality and simplification of a post-extraction process technology.