Residual carbon is a non-negligible component in biomass gasification slag and is seen as a factor related to the flowability of slag. One purpose of this study was to explore the relationship between residual carbon and rice straw ash fusibility via an ash fusibility temperatures (AFT) test in an argon (Ar) atmosphere. A visual measurement method for slag flow velocity was also established to further reveal slag flow properties. Furthermore, to illustrate the reaction mechanism of residual carbon with mineral components in ash, the surface morphology and chemical components of ash prepared at high temperatures were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the effects of residual carbon on rice straw ash fusibility characteristics were quantitatively calculated by thermodynamic equilibrium calculations. The experimental results showed that the increase in residual carbon mass fraction in ash increased ash fusibility temperatures and reduced slag flow velocity. The characterization and thermodynamic calculations indicated that in the ash fusion process, the rod-like structure containing carbon in ash disappeared, and the surface became compact due to the car-bothermic reaction with SiO2 with continuously rising temperatures. Then, the unreacted residual carbon and SiC could be completely covered by molten slag due to the further increase in the liquid phase caused by the melting CaMgSiO4 and CaMgSi2O6. The residual carbon in ash inhibited the formation of K2Si4O9, which further resulted in reduction of SiO2 migration to low melting silicate.