The adsorption isotherm equation published by Langmuir in 1918 describes the relationship between cumulative adsorption rate and adsorption pressure under isothermal condition. However，it should be noted that the Langmuir equation is an empirical equation of isothermal cumulative adsorption based on the experimental data from a large number of adsorption isotherm experiments by adsorption instrument（made by mica sheet）using methane（CH4），nitrogen（N2），carbon monoxide（CO），carbon dioxide（CO2），oxygen（O2），argon（Ar），and other gases. This equation has two constants，a and b.CHEN Yuanqian published the derivation of adsorption isotherm equation in 2018. There are also two constants A and B for this equation. The research results indicate that the two constants in the adsorption isotherm equations proposed by Langmuir and CHEN both have important physical meanings. The constant A of CHEN’s equation and the constant a of Langmuir equation represent the limit of cumulative adsorption rate of the sample. The constant B of CHEN’s equation and the constant b of Langmuir equation represent the decreasing rate of adsorption capacity. AB of CHEN’s equation and ab of Langmuir equation represent the maximum of theoretical initial adsorption rate of the sample. Meanwhile，the dimensionless CHEN’s and Langmuir adsorption isotherm equation and adsorption isotherm equation at instantaneous pressure were proposed，and a method for determining the isothermal saturated adsorption pressure and isothermal saturated cumulative adsorption capacity was proposed. The application of 16 cases shows that the limit of adsorption rate of samples evaluated by CHEN’s equation and Langmuir equation was basically same. For evaluations of the decline rate of instantaneous pressure adsorption rate and the maximum theoretical initial adsorption capacity，the results of Langmuir equation were significantly different from those of CHEN’s equation. However，it should be noted that Langmuir equation is empirical，and its reliability is lower than CHEN’s equation.