The effects of an industrial sludge (IS) on three high ash-fusion-temperature (AFT) coals (Jincheng coal (JC), Fuxing coal (FX) and Jiaozuo coal (JZ)) were investigated by an AFT detector, X-ray diffraction (XRD) analyzer, and thermo-physical modeling software. The results showed that IS could reduce the AFTs of three high AFT coals. The effect of IS on the AFTs of JC and JZ coals was clearly different from its effect on FX coal. When adding 18% IS ash to three coal ashes, the flow temperatures (FTs) of JC-IS and JZ-IS mixtures were reduced to below 1380 degrees C, which met the requirement of entrained-flow bed gasification, and the FT of FX-IS mixture was still higher than 1380 degrees C. After adding iron-rich IS ash (Fe2O3 > 41%) into three coal ashes, under reducing atmosphere, the Fe3+ was converted into Fe2+, the interactions of Fe2+ and Al2O3 led to the formation of a large amount of low melting-point (MP) hercynite (FeAl2O4). And some CaO could easily react with Al2O3 and SiO2 in the mixed ashes to produce anorthite (CaAl2Si2O8), restraining the formation of high MP mullite, which decreased the AFTs of these three mixtures. At 1100 degrees C, no mullite was detected in JC-IS and JZ-IS mixed ashes, and some mullite still existed in FX-IS mixed ash when the IS ash mass ratio even reached 30%, which might be the main reason for the decrease in the AFTs of JC and JZ was different from that of FX.