Abstract: The stripping effect and displacement effect are two key effects in ECBM technique(injected gas for enhanced coal bed methane recovery).In order to study the changing characteristic of the displacement effect, the characteristic values of the CH₄ partial pressure with the time step which occurs in the fracture system during ECBM has been refined.With the characteristic curve, the dynamic displacement adsorption experiment and the controlled experiment have been designed based on the the long-term displacement adsorption test methodology.It is found that the pressure gradient of CH₄ and the desorption enhancement of CH₄ by CO₂ are two basic functions in the displacement effect, by comparing two test results.The changing characteristics of two basic functions have been studied with test results.Finally, the mathematic form of the displacement effect has been built.It is shown that① 0.017 855 mol CH₄ has transformed into free state from adsorbed state in the desorption test, and 0.024 098 mol in the dynamic displacement adsorption test.The differences prove that CH₄ has been effectively promoted by strongly adsorbed gas CO₂.② The production rate of CH₄ which driven by pressure gradient, first increases at the initial stage and then decreases at the last stage.The production rate of CH₄ is proportional to the pressure difference between the CH₄ partial pressure with the initial adsorption pressure, and is inversely proportional to the slope of the characteristic curve(negative) of the CH₄ partial pressure.③ With the test results it is found that the desorption enhancement of CO₂ is concentrated in the initial stage and gradually decays as the test operates.With the first derivative function of EX-Langmuir function to the CH₄ partial pressure, it is found that the essence of the desorption enhancement of CO₂ is that CO₂ has an effect on reducing the adsorption capacity of coal for CH₄.As the increase of the CO₂ partial pressure, that effect has been constantly enhanced, but the extent of enhancement is decreasing.④ By combined the function of pressure gradient and the function of the desorption enhancement of CO₂,the dynamic displacement adsorption test result has been fitted, which exhibits a complex trend of down, rise, and down.The transformed relationship of two functions is the essential reason of the double peak of the fitted curve.