Abstract: In recent years, heat recovery technology has been applied in the field of modern coal gasification. Ash deposition on the radiant syngas cooler’s (RSC) heat exchange surface has become the bottleneck problem. Herein, this work aims to understand the deposit layer on the heat exchange surface at the RSC’s bottom. By characterizing the morphology, structure, and composition of the deposit layer, the ash deposition mechanism and its influence on the heat transfer process has been analyzed. The results reveal that the deposit layer can be divided into three main layers along the radial direction. The inner layer is a thin layer mainly composed of micro-particles. The middle layer is the sintered layer, being porous on the whole and dense at the micro-level. The large massive Fe-rich particles are supposed to be the connection points between the inner and middle layers. The outer layer is a molten layer with significantly fewer holes and dense lumps. According to the morphological characteristics, each main layer can be divided into two sub-layers. The content of Fe is higher on the inner side than on the outer side. The S content at the interface between the first inner layer and the second inner layer suddenly drops, which is related to the decomposition of iron sulfide. The contents of Na and K are enriched in the inner layer, decreased in the middle layer, and increased again in the outer layer. The NaCl and KCl crystal particles appear in the outer layer, so it is speculated that the temperature of the deposition location is higher than the melting temperature and lower than the condensation temperature of the alkali chlorides. Based on the deposit layer’s morphology and composition, the surface temperatures of three deposition states are estimated, and the heat transfer flux is calculated. The results show that the heat flux decreases by 25% when the inner layer is formed, and 40% when the three layers are all formed. The inner layer has the greatest thermal barrier effect, mainly related to its low radiation characteristics. This study indicates that the prevention of inner layer formation has important implications for developing efficient technology to control the RSC ash deposition. If the middle and the outer layers have already formed, the interlayer shear movement could cause the sedimentary layer to fracture and fall off from the weak point of connection.