Abstract: Based on the numerical analysis of COMSOL Multiphysics and similarity theory, we calculated the partial differential equations of gas radial instability of coal seam gas without un-simplified processing, and fitted the relation curve of lgY and lg F₀ derived by using Boltzmann function and derived a fitting equation. On the basis of the fitting equation, we derived the theoretical equation of permeability coefficient of coal seam through the variable substitution to solve problems that the permeability coefficient of coal seam can not find the proper calculation formula or there are two contradictory formulas in borehole radial flow method, and simplify the calculation procedure.