Abstract: In view of the importance of the permeability of top coal to the safety of the working face of fully mechanized caving mining, the variation of the permeability of top coal is studied and analyzed in the limit equilibrium zone. Vertical and horizontal strains are determined through damage mechanics and mathematical statistics with a mathematical model. The relationship between the volume strain and the permeability of coal is determined with the method of the triaxial compression test. The fluid used in experimentation is methane. The piecewise fitting is used to analyze the experimental data. The results show that according to the mathematical model for the volume strain, the volume strain at any point of top coal can be obtained. The horizontal strain of top coal is not related with the distance from the calculated point to the coal floor. The permeability at any point of top coal can be obtained from the relationship between the volume strain and the permeability. The relationship is obtained through test and fitting. The computing model is checked in practice.