Abstract: Underground coal gasification (UCG) is a clean technology for in-situ coal conversion with development potential, and numerical simulation is an effective means to study the complex gasification process. After nearly 50 years of development, UCG numerical simulation technology has achieved fruitful research results, but the technical development is still immature, which has become one of the main technical shortcomings restricting the industrialization of UCG. In order to accelerate the development of domestic UCG numerical simulation technology and better serve the field test, the framework of UCG numerical simulation technology are presented, the physicochemical behavior of gasification process is elaborated, and the main research progress and the latest achievements at home and abroad are summarized, and the existing problems and the directions for breakthroughs are pointed out. The study shows that: ① The gasification process involves many physicochemical reactions, large time and space span, complex heat and mass transfer processes, which lead to the challenging UCG numerical simulation technology, and the development of the world’s UCG numerical simulation technology can be divided into three phases, and numerical simulation technology has a certain lag compared with the field test, but the gasification process is not an unpredictable “black box”. ② The scientific essence of UCG is mass, momentum, heat transfer and chemical reactions in variable space, the prediction of gasification products and gasification cavity is one of the main tasks of numerical simulation, the expansion mechanism of gasification cavity includes chemical reaction consumption, coal spalling and roof collapse, the fluid flow in the gasification cavity is mainly controlled by double-diffusive natural convection driven by temperature and concentration gradients, radiation heat transfer dominates the heat transfer, and the permeability ratio of rubble to ash has an impact on the morphology development of the gasification cavity. ③ Packed bed model, channel model and coal slab model have advantages in product prediction, cavity prediction and drying front and combustion front tracking, respectively. The process model portrays the main physicochemical phenomena involved in gasification through different functional modules, which is a feasible strategy to realize the simulation of complex gasification processes, and the computational fluid dynamics model has developed rapidly in recent years and is the main simulation means at present. ④ In the future, the UCG numerical simulation technology needs to develop in the direction of more accurate, more systematic, more efficient and more intelligent, and urgently needs to solve the problems of large-size lump coal chemical reaction kinetics, multifunctional integration, the coupling of discrete element method and continuous medium method, the three-dimensional geological modeling of mine scale, and the integration of numerical simulation technology and artificial intelligence as soon as possible. With the continuous development and improvement of numerical simulation technology, it will certainly play a more important technical support role in the process of UCG industrialization.