In order to evaluate the slagging characteristics of lignite with high alkali and alkaline earth metal (AAEM) content under different atmospheres, three Xinjiang coals with similar AAEM compositions and various silica/alumina ratio (R_S/A) were used as fuels in this study, namely, QiYa1 coal (QY1c), QiYa2 coal (QY2c), and YiHua coal (YHc). The experiments were carried in a horizontal tube furnace system, and 3 atm of CO₂, CO/CO₂, and H₂O (g) were used to simulate gasification conditions in different parts of the boilers. Other than the characterization of samples, pseudo ternary phase diagrams and the thermodynamic modeling of ash melting behaviors were calculated using FactSage 7.2. Accordingly, the results demonstrated that the higher the R_S/A, the wider the distribution of the low-temperature region in the phase diagrams. Moreover, increasing the relative content of CaO was found to increase the ash melting temperatures of QY1c and QY2c. The proportion of the liquid phase QY1c and QY2c ashes in the CO/CO₂ atmosphere was observed to be much higher than that of CO₂ and H₂O (g) in a typical circulating fluidized bed (CFB) gasification temperature interval, while the generation of liquid phase YHc ash was less affected by the atmospheres. Additionally, the H₂O (g) atmosphere was found to promote the release of Na during the gasification process. Under the same atmosphere, coal with a higher R_S/A may more strongly capture Na-based species. The bottom of boilers was observed to be the most severe slagging zone, and purging with H₂O (g) served as a good method in alleviating the low-temperature slagging of the return feeder.