The co-combustion of sludge and coal in the existing coal-fired power plant is a promising way for the disposal of sludge. However, slagging in boilers is one of the potential problems if the sludge was mixed improperly with coal during co-combustion. In this paper, the ash fusion characteristics and mineral transformation behavior of a bituminous coal, an industrial sludge, a municipal sludge, and their mixtures have been studied. The results showed that the ash fusion temperatures of the mixed samples show a nonlinear relationship with the proportion of sludge. It was found that the variation of the ash fusion temperature of blended samples of coal with different sludges can be effectively predicted by a ternary phase diagram. Thermal analysis indicated that the decreasing of ash fusion temperature of the blends was caused by the melting phenomenon between the original minerals in coal and sludge which formed new minerals with a low ash fusion temperature. Mineral analysis showed that the formation of diopside, anorthite, and iron eutectic can reduce the ash fusion temperature of coal mixed with sludge, and the formation of merwinite, spinel, and mullite can increase the ash fusion temperature of the mixture.