Abstract: Underground Coal Gasification (UCG) is a highly promising technology for the clean and efficient utilization of coal. Its large−scale development is expected to effectively alleviate the contradiction between China’s resource endowment—characterized by being abundant in coal yet relatively scarce in oil and natural gas—and its growing demand for low−carbon energy. During the gasification process, coal is converted in situ into fuel gas, forming a gasification cavity. Its growth governs syngas production and resource recovery efficiency, and crucially determines operational safety and environmental impact. Therefore, elucidating the growth mechanisms and control laws of gasification cavity is a foundational prerequisite for achieving stable and efficient operation of in situ gasifiers. This paper reviews domestic and international research on the morphology, structure, and growth process of gasification cavities, systematically summarizes the factors influencing cavity growth, and clarifies their underlying mechanisms. Studies indicate that the gasification cavity generally exhibits a symmetrical morphology along the direction from the injection well to the production well, with the cavity gradually tapering radially toward the production well controlled by mass transfer. Axially, the cavity can be divided into the oxidation zone, reduction−pyrolysis zone, and drying zone based on reaction types; radially, it is categorized into the ash zone, dry coal zone, and wet coal zone according to differences in organic carbon content and moisture content. Cavity growth is essentially a complex outcome of heat transfer, mass transfer, reaction kinetics, and the thermo−mechanical behavior of coal rock, jointly influenced by process parameters and geological conditions. Furthermore, this paper summarizes common research methods for studying cavity growth and highlights numerical simulation as a crucial tool in current research. However, due to limited systematic understanding of the thermo−mechanical behavior of coal rock and a lack of reliable quantitative data and empirical equations, accurately predicting the actual morphology of the gasification cavity remains challenging. Future work should focus on systematically investigating the thermo−mechanical behavior of coal under various working conditions through physical simulation experiments, revealing its correlation with process parameters and geological conditions. This will provide theoretical support for accurately simulating cavity growth and achieving economical, safe, and efficient operation of in situ gasifiers.