Abstract: As an important cornerstone of my country's economic development and social stability, the clean and efficient conversion of coal is an important path to achieving sustainable development. As one of the core technologies for clean and efficient utilization of coal, coal gasification technology can not only effectively improve the utilization rate of coal resources but also significantly reduce environmental pollution. During the gasification process, most of the coal char particles directly participate in the gasification reaction in the gasifier, but some coal char particles will be deposited on the slag wall under the effect of gravity and complex airflow. These deposited coal char particles will continue to react with the gasifying agent near the wall, but due to the physical factors of slag flow and the chemical reaction factors of active minerals in the slag, some carbon cannot be completely converted and remains in the slag to form residual carbon. The formation of residual carbon in gasification slag not only affects the gasification efficiency but also affects the stable operation of the gasifier by affecting the rheological properties of the slag. Therefore, it is necessary to have a deep understanding of the formation mechanism of residual carbon during the deposition of coal char particles on the slag wall of the gasifier and its influence on the fluidity of the slag. To this end, the whole life cycle process of residual carbon generation was discussed in this paper from the influence of the environment in gasifier. First, this paper outlined the deposition behavior of coal char particles on the slag wall and explained the influence of factors such as wall temperature, wall melting state, coal particle carbon conversion rate, and coal particle mineral composition on particle deposition characteristics. Further, the formation mechanism of residual carbon during the deposition of coal char particles was elucidated by the deposition characteristic of coal char particles on the slag wall surface. Secondly, the physical and chemical properties of the residual carbon, such as the apparent morphology, microstructure, and chemical reaction properties, were analyzed, which are of great significance for the secondary utilization of residual carbon and improving the efficient utilization of carbon resources. Finally, this paper also summarized the effects of residual carbon addition on the melting characteristics and viscosity-temperature characteristics of coal ash with different chemical components. This review aims to analyze the formation of residual carbon in gasification slag from the source through multiple aspects.