The invention discloses a synthetic method of 3-methylvaleric acid and an application of the obtained product. The synthetic route comprises three-step reaction, the first step reaction is diethyl malonate synthesis, the second step reaction is alkalization, the third step reaction is acidification, and the 3-methylvaleric acid is obtained. According to the method, low-cost diethyl malonate, sodium ethylate and 2-bromobutane are used as raw materials for carrying out alkylation and decarboxylation. In addition, in the reaction process, organic solvents including benzene, diethyl ether and the like are not used for extraction, a novel extracting agent is used for extraction, and the possibility that finished products contain the organic solvents is reduced. In addition, the 2-bromobutane is more active than n-Butyl bromide, the yield can be improved, and the 3-methylvaleric acid with higher yield and content can be obtained. The obtained 3-methylvaleric acid can be used in tobacco flavor and sweet flavor production.