Coalbed methane resource is huge above 2 000 m in buried depth. An injection of CO₂ into coalbed can increase coalbed methane recovery, which can increase coalbed methane yield by 3.751×10¹² m³ above 2 000 m in buried depth. This paper, from multi-gas competitive absorption, coal volume expansion after CO₂ injection and its influence over penetrability and mechanics, and on-site tests, gives an introduction to its development progress worldwide. The on-site tests in several nations acquired similar results. This paper presents some factors needing consideration in study and on-site tests, such as the relation between multi-gas competitive absorption and coal molecular structure, absorption expansion volume and gas injection volume, relation between injection pressure and coal ranks, and safety issue by volume expansion after CO₂ injection into coalbed during mining.