When the adsorption experiment is carried out by using weighing method，the total mass mt at a test pressure point is composed by four parts，which are the mass of the test barrel mb，the mass of the core sample mc，the adsorbed methane gas mag and the free methane gas mfg in the void volume of the test barrel. And the mass of the injected methane gas mi only includes two parts，mag and mfg. Since the mass of the sample barrel and the mass of the sample loaded into the sample barrel are constant，this provides convenience for establishing a method for calculating the isothermal adsorption capacity by the weighing adsorption apparatus. Based on the principle of mass balance and Avogadro’s law，the method of calculating isothermal adsorption capacity using the data tested by the weighing adsorption apparatus was theoretically derived.Meanwhile，a criteria for judging adsorption degree，determining the saturated adsorption pressure and the saturated cumulative adsorption capacity was proposed. The application of shale gas and coalbed methane adsorption showed that the said method was correct and effective.