To control the stability of the roof of combustion space area(CSA),the concept of underground coal gasification(UCG)with shortwall mining method was put forward in this paper.According to the characteristics of the CSA roof in the coupled environment of high temperature and ground stress,we established a determining method for reasonable length of shortwall face of UCG and took a UCG project of a mine in Panjiang Group in Guizhou province as an example to calculate and analyze its reasonable length of shortwall mining.The results show that,with the advance of the flame working face,the temperature of rock mass near the bottom area(0~3 m)of the roof rapidly rises first and then gradually decreases,while the temperature of the rock mass at 3~10 m rises slowly,and the range of temperature conduction in overlying strata is about 10 m;the load and thermal stress of immediate roof of CSA rise rapidly after a period of stability,and the tensile strength decreases first and then rises,which leads to that the limit span of immediate roof decreases slightly first and then increase sharply;the reasonable length of shortwall face of UCG should be less than the minimum value of limit span of immediate roof,from which the reasonable length of shortwall face of Panjiang UCG project in Guizhou is identified as 12 m.This method could provide a theoretical guidance for the length design of UCG working face.