Isothermal adsorption equation and isothermal cumulative adsorption equation are two important theoretical equation that describe the adsorption rule of shale gas or coal bed methane. The former describes isothermal adsorption rate at different transient pressures，and the latter describes the cumulative isothermal adsorption rate curves at different pressures. It should be pointed out that the different data points on the isothermal adsorption curve are not directly obtained through experimental tests，but are obtained by theoretical calculation formulas. Although some methods for calculating the isotherm adsorption rate have been published at home and abroad，especially in China，incorrectness and uncertainty during the theoretical derivation process lead to errors in calculation formulas and calculation results，and the concept of excess adsorption was not correct，which make it impossible to provide reliable saturated cumulative adsorption data for the evaluation of adsorption gas resources of shale and coal bed. According to the gas state equation，using the principle of material conservation of gas，three methods for calculating isothermal adsorption rate were obtained after strict derivation. The judgment factor for determining whether the state of saturated adsorption rate is reached was proposed. The application of shale and coal bed adsorption experimental data shows that the new calculation method and judgment factor are correct and effective.