The geological conditions and coal seam characteristics have many differences in China. The complex formation has many characteristics, such as tectonic coal, high content of gas, high geostatic stress and the broken soft coal seam where plastic flow easily occur. Casing drilling technology is adopted to replace the traditional drill pipe for borehole gas extraction, the work efficiency and gas extraction rate were improved. It also can prevent many accidents, such as sticking and hole collapsing, and remove the bottom tool assembly without pulling the drill pipe out. Casing strength and bottom tool assembly were introduced and the application prospect of casing drilling technology in complex formation of coal mine is discussed.