Slag behaviors on the top cone are significant characteristics that relate to the refractory wall corrosion and safe operation in a membrane wall gasifier. The slag flow and heat transfer model was established to predict the slag behaviors on the top cone of a gasifier. The results showed that the closer to the top outlet of the gasification chamber, the thinner the slag thickness, the higher the slag layer heat flux density and refractory wall surface temperature. The mean slag thickness of the top cone was thinner and the mean heat flux density of the top cone was higher than that of the bottom cone, thus the protection of the upper refractory wall was weakened. Especially, the heat transfer rate of the overall top cone significantly decreased when the cone angle increased. Moreover, the refractory wall corrosion model was built based on the active oxidation behavior. The results showed that the refractory wall surface temperature and linear corrosion rate significantly increased, while the gas temperature increased.