The invention provides a small-molecule inhibitor with the structural formula as below, wherein R1 represents imino group, carbonyl, furan or thiophene, R2 represents hydrogen, hydroxyl, methyl, ethyl, phenyl or benzyl, and R3 represents hydrogen, nitro, amino or hydroxyl. The inhibitor also includes a pharmaceutically acceptable salt with a chemical formula shown as the general formula I. A piece of AutoDockVina software is used to connect the small molecule comprises the above skeleton and substituents to a determined AdoMetDC pocket, and the small molecule is evaluated by the scoring given by the AutoDockVina software. Small molecule with lowest scoring less than 7.0 is used for experimental verification; and the above small-molecule inhibitor is applied to the inhibition of S-adenosylmethionone decarboxylase activity.