The invention discloses a synthetic method for high-purity astaxanthin. The synthetic method comprises the following steps: dissolving dehydroastaxanthin in an organic solvent, then adding water, zinc powder and a phase transfer catalyst, then dropwise adding glacial acetic acid and carrying out a reduction reaction, and after the reaction is completed, carrying out postprocessing so as to obtain astaxanthin. According to the invention, by adding a certain amount of the water and the phase transfer catalyst into a reaction system, the synthetic method reduces side reaction, improves the yield and the purity of the reaction, and effectively reduces the usage amount of glacial acetic acid.