The invention provides S-2-hydroxy-3-methoxy-3, 3-dibenzylpropionic acid and a preparation method thereof. The preparation method comprises 1, dissolving 2, 3-
epoxy-3, 3-dibenzylpropionate in
methanol, carrying out acid-catalyzed ring-opening,
alkali metal hydroxide hydrolysis and reaction solution condensation, adding water into the condensed solution, filtering the solution after solids are separated, and carrying out
solvent washing and purification to obtain refined 2-hydroxy-3-methoxy-3, 3-dibenzylpropionate, and 2, carrying out a reaction process on the refined 2-hydroxy-3-methoxy-3, 3-dibenzylpropionate and methyl L-prolinate, carrying out splitting to obtain methyl S-2-hydroxy-3-methoxy-3, 3-dibenzylpropionic acid-L-prolinate, adding an acid into the methyl S-2-hydroxy-3-methoxy-3, 3-dibenzylpropionic acid-L-prolinate so that the methyl S-2-hydroxy-3-methoxy-3, 3-dibenzylpropionic acid-L-prolinate is free and carrying out extraction,
drying, condensation and recrystallization to obtain S-2-hydroxy-3-methoxy-3, 3-dibenzylpropionic acid. The preparation method has simple processes, can produce high purity S-2-hydroxy-3-methoxy-3, 3-dibenzylpropionic acid, can guarantee
ambrisentan reaching the standard, can shorten a
synthesis period, is free of an acid binding agent in a splitting stage and reduces a cost.