Abstract: The air tightness of the overburden strata was one of the conditions to determine the resources quantity of the coalbed methane. With the analysis on the formation cause and properties of the mudstone overpressurized layer, a mathematical equation of the coalbed methane reservoir seepage was established and the con straint conditions of the coalbed methane migration under the stable geological condition was discussed. The study found that the mudstone overpressure formation ha d stages and was controlled by the variation of the geological condition. The overpressure reservoir would reach an critical depth condition for a certain pressure gas se aling. With the depth increased and the ground temperature increased, the decreased porosity of the overburden strata would cause the overpressure variation, the ove rburden strata affected to the adsorption resistance of the pore water and the replacement pressure of the bottom strata also would be changed and the sealing capacit y of the overburden strata to the coalbed methane would be steadily improved.