Abstract: The gas ad/de sorption induced swelling and shrinkage strain in coal are important factors that influence the dynamic change of coal permeability during coalbed methane (CBM) extraction.In this study,the middle and high rank coal samples of Permian in eastern Ordos Basin and southern Qinshui Basin were selected.A series of experimental measurements on the sorption induced strain using a desorption seepage testing system was carried out.The adsorption strain values of middle and high rank coals were measured,and the method of calculating adsorption strain by fitting the 12-24 hours’adsorption strain data points was proposed.The differences of adsorption induced strain and permeability evolution between high and middle rank coals were analyzed.The law of adsorption induced strain and its influence on the permeability of medium and high rank coals and both control mechanism were revealed.The results show that the relationship between methane adsorption strain and equilibrium pressure in middle and high rank coal samples conforms to the Langmuir isothermal adsorption model.The Langmuir radial strain,axial strain and volumetric strain induced by the CH4 adsorption of middle rank coal are 0.18×10-2,0.09×10-2 and 0.44×10-2,respectively and the corresponding Langmuir pressures are 5.26,6.93 and 5.42 MPa.The Langmuir radial strain,axial strain and volumetric strain induced by CH4 adsorption of high rank coal are 1.91×10-2,0.22×10-2 and 3.99×10-2,respectively,and the corresponding Langmuir pressures are 7.87,4.57 and 7.49 MPa.At a constant effective stress,with the increase of adsorption equilibrium pressure of coal,the adsorption induced strain increases,and the adsorption induced strain of high rank coal is obviously higher than that of mediumrank coal.The gas adsorption induced strain in coal is mainly controlled by the gas adsorption capacity in coal.Meanwhile,at a constant effective stress,the permeability of middlerank coal is higher than that of high rank coal.With the increase of adsorption equilibrium pressure,the permeability declines by a negative exponential function of different rank coals.The rate of permeability reduction of high rank coal is higher than that of low rank coal.In the process of CBM drainage and production,after the reservoir pressure declines to below a critical desorption pressure,the effect of CBM wells production improvement of highrank is better than that of middle rank.