Abstract: In order to deeply study the mechanism of injecting gas to replace coal-bed CH₄ technology,on the basis of studying gas injection to replace coal-bed CH₄ mechanism at home and abroad,five main aspects of injecting gas to replace coal-bed CH₄ mechanism are achieved,they are,the replacement action mechanism of injecting gas,the action mechanism of reducing CH₄ partial pressure,the action mechanism of sieve adsorption replacement,the action mechanism of improving the coal-bed CH₄ diffusion-seepage velocity and the action mechanism of fracturing antireflection.The existing problems of research in injecting gas to replace coal-bed CH₄ technology mechanism are analyzed,the direction of the technology is discussed.