Abstract: The fusibility and viscosity-temperature characteristics of municipal solid waste (MSW) ash slag are important to the optimize design and operation of fixed bed slag gasifier. In this paper, the ash composition characteristics of two MSW samples were analyzed, and the melting mechanism of MSW ash was explored by high temperature heating stage microscope (HTSM), X-ray diffraction (XRD) and FactSage. At the same time, the effect of crystal mineral formation on ash viscosity was analyzed by high-temperature viscometer, SEM-EDS and XRD. The results show that the silica to aluminum ratio of MSW ash are both high, but the contents of aluminum and calcium are different. The flow temperature of YZ ash is about 150 ℃ higher than that of LG ash. Low temperature eutectic of wollastonite is the reason for the lower melting point of LG ash, and the existence of quartz and spinel at high temperature leads to a higher melting point of YZ ash. Ash melting behavior of LG and YZ are both "melt-dissolution" mechanism, and they both undergo shrinkage, melting and diffusion processes with the increase of temperature. The viscosity-temperature curves of LG and YZ are both glassy slag. However, the viscosity-temperature characteristics of YZ ash are relatively poor, which is related to the formation of anorthite during the cooling process. So the application of YZ requires a high slag discharge temperature. LG ash has good fusibility characteristics and viscosity-temperature characteristics, so the gasifier with this material can operate in a wide temperature range.