Abstract: During Underground Coal Gasification (UCG), the composition and evolution of solid−phase pollutants in the target coal seam area−comprising the dry distillation−drying zone and the reduction zone−significantly influence environmental outcomes. In this study, a tube furnace was used to simulate the reaction conditions in these zones, and comparative pyrolysis−reduction experiments were conducted. X−ray diffraction (XRD) was systematically applied to investigate mineral phase transformations and the occurrence forms of heavy metals in solid pollutants. The main findings are as follows: (1) In the dry distillation−drying zone, solid pollutants are dominated by kaolinite, quartz, and amorphous carbon. Elevated temperatures (above 500 ℃) promote the dehydroxylation of kaolinite, converting it into amorphous aluminosilicates, while heavy metals such as lead (Pb) and manganese (Mn) remain primarily in oxide forms. (2) In the reduction zone, the presence of wüstite (FeO), metallic lead (Pb⁰), and lead sulfide (PbS) indicates a strongly reducing environment that facilitates the reduction and sulfidation of heavy metals. (3) Pyrolysis temperature critically affects the mobility of heavy metals: Pb exists mainly as PbS at low temperatures (300 ℃), whereas the crystallinity of metallic Pb⁰ increases significantly at 700 ℃. Moreover, manganese (Mn) and copper (Cu) exhibit higher mobility during pyrolysis, while zinc (Zn) and cadmium (Cd) show lower mobility. By elucidating mineral transformation pathways and heavy metal immobilization mechanisms through XRD, this study provides a scientific basis for predicting pollutant behavior and developing in situ mineral carbonation sequestration strategies for UCG.