Based on the study on the gas diffusion mechanism in subground rocks and the method measuring gas diffusion coefficient, This paper points out the difference between measured gas diffusion coefficient and that in the stratigraphic condition, by using the gas diffusion coefficients in the condition of no water and  water-saturated, established  the relation between the ratio of the gas diffusion coefficients of man-made quartz silt-stone in no watersaturated and in water condition, by this relation, this paper establishes the method of medium change to measured gas diffusion coefficient.  By Einstein equation, this paper established the method of temperature correcting to measured gas diffusion coefficient. The applied results of this method indicate that it tallies with geological law and prove that it is practical.