Underground coal gasification (UCG) is a highly efficient new type of coal mining technology with broad future prospects. In order to study the cavity extension formation in the early ignition stage of UCG, a block coal scale UCG simulation experiment was carried out. The results show that after the ignition, the temperature above ignition point rose fastest, and the coal combustion interface and high temperature area moved toward to the above of ignition point, while the temperature of the left and right sides of ignition point rose a little slowly. According to the results of dissected block coal, it is indicated that the extension scale in the vertical direction was significantly larger than other directions; the combustion cavity form was an irregular rectangle like a pear. The results of this experiment revealed the cavity extension process from ignition of UCG channels to the formation of cavity, which provided a foundation for the study of extension characteristics of UCG channel in the entire UCG process.