Red mud is a solid waste residue from the caustic soda leaching of bauxite ores to produce alumina by the Bayer process. Red mud contains large quantity of alkali and aluminosilicate, so it is potentially available to prepare inorganic polymeric materials by geopolymerisation process. However, the activity or dissolubility of the aluminosilicate phases in red mud is significantly poor, which constraints the geopolymerisation process. Therefore, some pretreatment process for red mud is necessary to improve the adhesive property and dissolubility of Bayer red mud. In this study, mineral phase transformation and dissolubility of a typical red mud sample were studied under different thermal treatment processes. The thermal behavior of the red mud was studied by TG-DTA. The crystalline phases of the samples calcined at 200-1000 °C for different hours were determined by XRD, and the dissolubility was determined by alkaline leaching test. The TG-DTA pattern shows no obvious endothermic or exothermic peaks, and the weight loss increases continuously as the temperature rises, which indicates that the crystalline phases transform continuously as the temperature rises, consistent with the XRD results. As the calcination temperature rises from 200 to 800 °C, several kinds of crystalline phase in original red mud, including gibbsite, katoite, muscovite, natrodavyne disappeared in succession, accompanied with the formation of nepheline, gehlenite, sodium aluminum silicate, and some amorphous aluminosilicate. The calcined products are more likely to dissolve. But when it rises over than 800 °C, the content of gehlenite increases, and the phase of which is stable. As the calcination temperature rises from 200 to 1000 °C, the dissolubility of aluminosilicate in the red mud under high alkaline conditions increases firstly and then decreases after over 800 °C. Therefore, the optimum temperature of thermal treatment for red mud is about 800 °C. This study could contribute to the following preparation of geopolymeric material made from red mud, especially the pretreatment process of red mud.