Moisture adsorption properties of dried lignite were investigated experimentally and mathematically. Three kinds of lignite (Hailaer, Huolinhe, and Indonesia) with different water contents were tested with a machine that maintained a constant temperature and relative humidity. The obtained experimental adsorption data were applied to the Guggenheim, Anderson, and de Boer (GAB), modified Guggenheim, Anderson, and de Boer (MGAB), modified Oswin (MO), modified Henderson (MH), and modified Freundlich (MF) isotherm equations to test their applicability to dried lignite. The order of best fit of adsorption across the entire temperature and relative humidity range was MF > MGAB > GAB > MH > MO. On the basis of the experimental data, the MF equation was modified to consider the effect of the initial moisture content of dried lignite on the equilibrium water. The critical temperature and relative humidity values were obtained with the modified MF equation, which was then used to determine the moisture adsorption occurrence for the dried lignite with a specific moisture content under different environmental conditions. Shanghai and Guangzhou were the target-user locations of the dried lignite. The critical water content values were obtained to avoid the moisture adsorption. These values provided the theoretical guidelines for the target control of the lignite drying process and storage conditions.