The decarboxylations of the intermediates involved in the industrial production of furanone starting from diethyl α-methyldiglycolate and diethyl oxalate have been examined. Furanone is obtained through two successive Claisen condensations, hydrolysis of esters, methylation, and decarboxylation in this process route. During the reaction, both ester groups of diethyl α-methyldiglycolate are removed during the process, but the approaches of two decarboxylations are obviously different. After the Claisen ester condensation, the first ester group is removed in the form of a carbonate by an addition-elimination mechanism in the presence of CH3ONa; the second ester group is removed in the form of CO2 through a sodium carboxylate intermediate under basic conditions following the methylation. In order to verify the generality of decarboxylation of the intermediate: the decarboxylation of β-carbonyl ester occurred without hydrolysis under basic conditions. The alkylation of α-ethyl acetoacetate was carried out to observe whether the product (α-ethyl-α-butyl acetoacetate) would undergo decarboxylation. However, the expected decarboxylation product of α-ethyl-α-butyl acetoacetate (3-ethyl-2-heptanone) was not observed. Therefore, the results indicated that common β-carbonyl esters could not undergo decarboxylation attacked by a base through an addition-elimination mechanism. The driving force for the two abnormal decarboxylation in the preparation of flavor furanone is due to the formation of stable aromatic conjugated structures after decarboxylation. This research has clarified the reaction mechanism of the preparation process of the flavor furanone for the first time, which will be helpful for the improvement and optimization of the technical route.