This paper aims to explore the preparation method and reaction mechanism of erythro aleuritic acid from thero configuration. Erythro aleuritic acid was prepared by configuration transformation method from thero aleuritic acid. And FTIR, mass spectrometry, nuclear magnetic resonance (1H/13C NMR), TG, DSC, XRD, chiral resolution and optical rotation were used to analyze and confirm the structure of reaction products. On these foundations, the transformation mechanism of thero aleuritic acid to erythro aleuritic acid was primary explored. The results showed that the structure of the product was consistent with thero aleuritic acid by FTIR, mass spectrometry. The results including NMR. XRD, DSC and TG show that the thero aleuritic acid and the product have the same crystal type, but the melting point of the product is higher and the crystal cell structure is smaller than thero configuration. The chiral resolution results showed that the product was an optical isomer of aleuritic acid, which was determined to be erythro aleuritic acid. Based on the structural characterization of the intermediates, the mechanism of the transformation from thero aleuritic acid to erythro aleuritic acid is the simultaneous reaction of SN2 and E2 between the halogens and the hydroxides. The twice SN2 reactions are the key to the configuration transformation reaction (Walden transformation). The E2 reaction of hydroxyl/halogen is the reason why thero aleuritic acid can be completely transformed into erythro aleuritic acid.