Under the multifield coupling of temperature, crustal stress and gas pressure, a coal-gas experimental device with microscopic observation has been developed to reveal the significance of the coal-gas reaction for the extraction of coal-mine gas. ECBM and the sealing of carbon dioxide in deeper coal seam, and to observe more accurately the dynamic process of coal-gas interaction under multifield coupling from a micro perspective. The microscopic-observing experimental device consists of sealed container and clamping device, pressure system, temperature-field system and microscopic-observing system. Combining with image processing techniques, the device is able to directly observe the slight changes caused by the interaction between coal and different kinds of gases under multifield circumstances. After performing adsorption-swelling deformation experiment of gas and carbon dioxide under conditions of no-stress loading, the same temperature and air pressure. Results show that: (1) The sample experienced three deformation stages in the two gases about rapid growth, slow growth and balance, which is similar to the adsorption isotherm curve. (2) The sample use time to achieve saturation adsorption in CO2 condition is shorter than it in methane. (3) The sample’s maximum longitudinal stain in CO2 condition is about 0.5%, which is the three times of the value in methane at the same conditions (0.15%). It indicates that the device has features of simple structure, high visibility and accuracy.