The invention discloses a monoamine oxidase A inhibitor, which is obtained by using cheap ruthenium carbon as a catalyst to catalyze the synthesis of thioether at room temperature, specifically: using the compound represented by formula (III) as the reaction raw material, Ru / C as catalyst, NaIO 4 is an oxidant, water is a solvent, and the reaction is carried out at room temperature. After the reaction is completed, the reaction liquid is post-treated to obtain the sulfone compound shown in formula (IV). The method of the invention uses water as a solvent, oxidizes cheap ruthenium carbon with sodium periodate to generate ruthenium tetroxide, and further oxidizes the sulfide compound to synthesize the target product, and the preparation method is more efficient, quick, environmentally friendly and mild. The prepared monoamine oxidase A inhibitor has good monoamine oxidase A inhibitory activity, can be used to prepare a drug for inhibiting the activity of monoamine oxidase A, and has broad application prospects in drug development, in order to provide a more effective way for the treatment of related diseases.