Four slag samples of Shenmu bituminite and Huolinhe lignite (S-SM(A), S-SM(B), S-HLA(A), and S-HLH(B)) selected from two upper parts of a multistage conversion integrated fluidized bed (MFB) were analyzed by X-ray fluorescence spectrometry, scanning electron microscopy and X-ray diffraction analyses. The results show that the ash sintering temperature and ash fusion temperature of slag samples are below those of the raw coal ashes and increase in the order S-SM(A) < S-SM(B) < S-HLH(A) < S-HLH(B) because of differences in acid/base composition. The slag samples are composed of amorphous matter and sortie crystals, and the crystal content of the two S-HLH samples are higher than that of the two S-SM samples. The section of the MFB above the site of Oxygen injection may be divided into three zones based on particle movement. Fine mineral particles contain much more iron and calcium than other particles. The possible mechanism of slag formation may involve mineral exposure by char gasification, interaction of minerals, formation of particles of different sizes, formation of an initial adhesive layer, and emergence of slag.