The invention relates to a coalbed methane mining technology, aims at solving the problem of drilling fluid loss of a goaf in coalbed methane extracting and deep drilling processes, and provides a casing drilling technique for an over-goaf. The casing drilling technique comprises the following steps: driving a first collaring head to drill through a finished drill-hole which is 10m below a weathered bedrock by virtue of a drill stem; arranging a casing on a surface layer of the next open pore to the bottom, and carrying out well cementation with cement; adding cement and returning to the earth surface; driving a second collaring head to drill into a coal seam floor of the goaf by virtue of a casing instead of the drill stem, reserving the casing into a pit shaft as an intermediate casing after second collaring is ended, and carrying out well cementation on the position between the intermediate casing and the borehole wall of the goaf; further driving a third collaring head to drill into a finished drill-hole which is 30m below the next group of coal seam floors by virtue of the drill stem. According to the casing drilling technique, a ground casing drilling technique for preventing leakage loss is adopted, when a drill drills into the coal goaf, heavy leakage loss caused by the goaf is lowered by taking the casing as a liner tube; second casing drilling or multiple casing drilling is carried out; drilling of finishing drilling by taking the next group of coal as a target layer through the goaf is achieved.