Conversion of lignin into value-added chemicals is attracting growing attention due to the depletion of fossil fuels and the abundant resource of lignin. In this study, hydrothermal conversion of a model compound of lignin, catechol, into value-added four-carbon dicarboxylic acids (C4-DCAs), such as tartaric (HOOC-CH(OH)CH(OH)-COOH), malic (HOOC-CH2CH(OH)-COOH), and fumaric (HOOC-CH=CH-COOH) acids was investigated. The yield of total C4-DCAs can reach as high as 41.0%, and alkali played a key role in not only promoting the production but also avoiding the decomposition of C4-DCAs. The reaction mechanism of hydrothermal conversion catechol into C4-DCAs showed that catechol is first oxidized to o-quinone, which is then attacked by the hydroxyl radical (OH) or the hydroperoxyl anion (HO2) via conjugate addition to decompose into C4-DCAs. This result is helpful to facilitate studies for developing a new, green, and sustainable process to produce value-added C4-DCAs from lignin.