The efficient utilization of abundant low-rank coals, which show high moisture and ash content, is significant for coal-based energy systems. The gasification of Zhaotong lignite was performed with different alkaline-metal catalysts, such as KOH, NaOH, K2CO3, and Na2CO3, in supercritical water. The behavior of the alkaline metal was also investigated. Potassium shows better catalytic activity than sodium. KOH not only promotes breakage of chemical bonds in the coal matrix to generate low-molecular-weight gases, but also alters the pore structure of the char to improve the gasification of carbon in the residue. However, potassium is partly inactivated upon reaction with minerals in coal in supercritical water gasification. In situ protection of potassium is realized by adding Ca(OH)(2) under supercritical conditions.