Sulfur-containing flavor compounds have the characteristics of low threshold and strong specificity, and have important contributions to the characteristic aroma of many foods. The α-methylthiomethyl cinnamaldehyde with FEMA No. 3717 is a sulfur-containing flavor compound with typical broccoli and green broad bean aroma. Due to the limitation of preparation methods, the flavor had not yet achieved large-scale productions and literature reported that its yield was only about 8%. Using cinnamaldehyde as the raw material, dimethyl sulfoxide and oxalyl chloride as methylthiomethylation reagents, a new method of flavor α-methylthiomethyl cinnamaldehyde was developed. GC-MS was used to monitor the residual amount of cinnamaldehyde in the reaction, and the impact of reaction conditions such as the reaction ratios, solvent type, reaction temperature on product yield was examined. The results showed that, with xylene as solvent, dimethyl sulfoxide and oxalyl chloride reacted first to produce an intermediate of electrophilic methyl methylenethionium salt, and then triethylamine, 1,4-diazabicyclo [2.2.2] octane (DABCO) and cinnamaldehyde were added to the reaction system. Under reflux conditions, the nucleophilic enol anion produced by the conjugation of DABCO and cinnamaldehyde reacted with methylenethionium salt ion. α-Methylthiomethyl cinnamaldehyde was obtained after elimination of addition product, and the yield could reach 52% under the optimized conditions. The stereoconfiguration of product α-methylthiomethyl cinnamaldehyde was detected by the NOE difference spectrum method of ¹H NMR. The double bond in the molecule has been confirmed to be of the E configuration. The research is anticipated to provide valuable references for large scale production of flavor α-methylthiomethyl cinnamaldehyde.