The invention belongs to the field of synthesizing medicine intermediates, in particular to a preparation method for a trans-4-alkylthio cinnamic acid (I). The invention uses a cinnamic acid and a chlorosulfonic acid as raw materials to firstly prepare a trans-4- chlorosulfonyl chichi cinnamic acid, then uses a stannous chloride-acetic acid system reduction thrans-4-chlorosulfonyl cinnamic acid to get an unstable intermediate trans-4- mercapto cinnamic acid. The latter is not needed to be separated, which can be directly reacted with an aliphatic (alkyl) halogenated hydrocarbon in an alkalineaqueous liquid to synthesize the needed products. The trans-4-alkylthio cinnamic acid synthesized by the method is an important medicine synthesizing intermediate and can be used to synthesize natural products with physiological activity, which can obtain the cinnamic acid containing a sulfoxide group and a sulfonyl group through selective oxidation and is a valuable 'block' in organic chemistry synthesis. After the reaction with Hunsdiecker or the decarboxylation reaction by adding bromine, the trans-4-alkylthio cinnamic acid can also be converted to a Beta-brominated aromatic ethylene, which is widely applied to synthesize the selective stereo substituted olefins with the existence of metal catalytic reagent.