Abstract: In order to clarify the mechanism of driving out gas in the coal seam， the soft coal sample of the No.2 coal seam structure was collected in the Xinggong coalfield of western Henan， and the large-scale tests on CO₂ injection was performed to drive out coal seam gas injection in laboratory. Results show that the CO₂ injection of driving out the gas in the coal seam is a dynamic process， combined with the knowledge of domestic and foreign experts on the gas injection mechanism of coal seam gas displacement， the definition of replacement effect and displacement effect is given. Based on the change of CH₄ concentration in the gas outlet， the gas injection process was divided into three stages to study the conversion process of CO₂ promoting CH₄ mechanism in coal seam. Before CO₂ breaks through the cavity， it mainly shows the replacement effect. The replacement effect of CO₂ injection is dominant. After CO₂ breaks through the cavity， part of the injected CO₂ is replaced with CH₄， and part of it and CH₄ flow out of the cavity， with a concurrent state of replacement and displacement effect. For the whole experimental process， the CO₂ replacement effect is more obvious and plays a leading role.

Key words: CO₂-ECBM, promote gas drainage mechanism, timeliness, displacement effect, replacement effect