Abstract: The ash fusion characteristics of coal mixed with different kinds of sludge were investigated. Mineral ternary phase diagram and XRD analysis were used to identify interaction mechanism of different mineral components and transformation of ash melting characteristics during burning of coal with municipal and industrial sludge. The results show that ternary phase diagram can effectively predict the variation of ash melting temperature for co-combustion of coal and sludge. When the content of iron oxide is low, the formation of eutectic, diopside and anorthite can reduce the ash fusion temperature. Monticellite, mullite and monomer form of iron oxide improve the ash fusion temperature. The sulfur in municipal sludge easily forms low temperature eutectic of sulfate. The influence of phosphorus on the ash fusion temperature is related to the ratio of alumina and alkali. When the alumina is dominant, phosphorus will reduce the ash fusion temperature. When the alkali is primary, phosphorus will raise the ash fusion temperature.