Abstract: Integrating sludge with coal-fired units can overcome the difficulties of sludge combustion and its low calorific value, while also reducing coal consumption. To elucidate the mechanism by which sludge co-combustion affects the ash fusion characteristics of lignite, lignite and sludge were selected as the research materials. The ash fusion temperatures at different blending ratios were measured, and mineral phase transformations and liquid phase formation were analyzed using X-ray diffraction (XRD) and FactSage simulations. The experimental results show that as the sludge blending ratio increases, the fusion temperatures of the mixed ash gradually decrease, with the deformation temperature dropping markedly from 1237 °C to 1124 °C. The results of XRD analysis show that the low-melting-point Ca-Mg phosphates and K-Fe silicates are formed by the reaction of P and Fe components in sludge with alkali metals/alkaline earth metals in coal and sludge itself in the low-temperature melting stage, which is the main reason for the decrease of fusion temperatures of mixed ash. In the high-temperature melting stage, alkali metals/alkaline earth metals further promote the formation of low-temperature eutectics of feldspar minerals and reduce the ash fusion temperature. Thermodynamic simulation shows that the ash deformation temperature corresponds to the onset temperature of liquid phase formation, while the flow temperature corresponds to the temperature at which the liquid phase content reaches 75%. In the low-temperature melting stage, low-melting-point phosphates and silicates dominate the initial melting, which tends to aggravate slagging. In the high-temperature melting stage, due to the low-temperature eutectic reactions, the mixed ash rapidly reaches a high liquid phase content at the flow temperature. The results of this study provide a theoretical basis for the rational co-combustion of sludge and the prevention of boiler slagging in coal-fired power plants.