The key issue of enhancing coalbed methane recovery by gas injection is reservoir permeability variation, which is controlled by gas-induced coal swelling. The research finding on coal swelling were collected and analyzed, and it was found that there exist disagreements, which is due to the lack of understanding about swelling mechanisms and different experimental methods. Compared with dilatometer and strain gauge, optical method is more accurate when measuring coal swelling. However, in future work, larger block should be adopted to represent coal reservoir more effectively. When conducting research on coal swelling mechanism, more attention should be paid to the original mechanical property of coal macromolecular structure and the impact on coal cross-linked network of CO₂.