Abstract: As an in-situ chemical coal mining method,underground coal gasification(UCG) is listed as the strategic direction of innocuous coal mining technology in the Action Plan of the National Energy Technology Revolution & Innovation(from 2016 to 2030)in China.This paper summarizes the progress of UCG from two aspects of theoretical research and technological development.The researches related to UCG cavity growth and its influencing factors,and the movement of the coal seam overburden under the condition of thermal mining are systematically described.Taking the process route as the main line,this paper summarizes the development of UCG technology,introduces the characteristics of process route,connection of gasification wells,configuration of the gasifier,operation mode of gasifier and typical field test cases.The modern UCG is based on the controlled retraction injection point (CRIP) technology,which integrates modern drilling technology,advanced petroleum equipment and downhole measurement technology.The gasifier is mainly composed of long-distance directional boreholes.Through the remote controllable multi-phase integrated injection & measurement equipment,the precise control of coal seam fire face and the real-time control of operation parameters could be realized.This technology has obvious advantages in in-situ conversion of deep coal seams,however,it is necessary to promote the industrialization of deep UCG orderly based on the comprehensive evaluation of the pilot project.