Abstract: The law that the adsorption values available for coal Langmuir isotherm testing isn't constant but variable under the same testing conditions occasionally was discovered by the author at studying coal absorption capacity.The experiment result indicates different maximum pressures used in the testing cause the variation of absorption value.The research result further reveals that the relation between absorption constant V_L & p_L and maximum testing pressures is positive linear trend,and this law identified is of universal based on a lot of absorption data of coal samples from several main coalfields of China.In order to show the correlation among absorption constants and make full use of the data and information collected from the past experiments,a special method transferring absorption estimates under different maximum testing pressures to same maximum pressures is established in the testing study.