To investigate fusibility characteristics of residual ash from Xiaolongtan lignite (XLT) pressurized fluidized-bed ash agglomerate (PFBA) gasification and its formation mechanism, the ash samples were examined by press-drop sintering technique, scanning electron microscope with energy-dispersive X-ray detector (SEM-EDX), and X-ray diffraction (XRD) analyses. The results show that both sintering temperatures and ash fusion temperatures (AFTs) of ash samples decrease from the gangue ashes to agglomerates to slag, because of the increase of the total base content and the differences in their mineral compositions accordingly. The surfaces of the slag were obviously molten; the agglomerates were covered with some small spheres and large particle aggregates, and the gangue ashes were composed of some irregular prismatic particles. During XLT PFBA gasification, the agglomerates originate from the collision and reunification of low-melting minerals under the dragging force of rising gases and the gangue ashes result from some thermally stable mineral particles with a high-melting point. When the PFBA gasification deviates from the suitable operation parameters (e.g., the gas velocity of the distribution plate is low, the gas velocity of the tubular loop is high, or the steam/oxygen ratio is low), the slag might be formed in the gasifier.