By blending municipal sewage sludge with coal to prepare sludge slurry, it is expected to achieve resourceful, harmless, and re⁃ducing disposal of sludge by using entrained-flow gasification technology. At present, there are few studies on the influence and mecha⁃nism of municipal sludge on the viscosity-temperature characteristics of gasification coal ash, which limits the application of sludge-coalwater slurry in gasification. Ash of typical coal (YLA) for gasification was selected and mixed with different proportions of sewage sludgeash (SSA) to simulate the co-gasification ash of sludge and coal, and the ash fusion temperatures and viscosity-temperature curves of theraw and mixed ash samples were measured. XRD analysis and phase diagram theory were used to investigate the variation of crystallizationtendency in slag during the cooling process, and FTIR was used to characterize the structural distribution of slag silica-oxygen units and toelucidate the mechanism by which municipal sludge ash content changed slag viscosity. The results show that 10% is the ideal SSA addi⁃tion ratio and the slag discharge performance will be deteriorated if SSA content is above 30%. When SSA content is 10%, the flow tem⁃perature is sufficiently reduced and has an operable temperature range of 168 ℃ . When the SSA content exceeds 30%, the flow tempera⁃ture increases due to the significant increase in iron and phosphorus content in the ash, and the operable temperature range is narrowed.And the critical viscosity temperature is close to the lower limit of the operating temperature due to the increased crystallization tendency,which easily causes drastic changes in slag viscosity. FTIR analysis and peak fitting results show that the reduced polymerization of the sili⁃ca-oxygen structure is the main reason for the decreasing viscosity of the slag with increasing SSA content.