Abstract: Adsorption constant derived from adsorption isotherm is a fundamental indicator in coal mine gas control, CBM extraction and shale gas development. However,the current experimental studies have obtained two types of adsorption isotherms with different characteristics:Langmuir type and Central-peaked type. Using Langmuir model to fit non-Langmuir type isotherm not only results in great errors in adsorption constant,but also brings great production safety risk. In this regard,by deducing the mechanism of adsorption test,isothermal adsorption test,correction of ad- sorption isotherm and error analysis,the following understandings have been obtained:① Isothermal adsorption test can only measure the “peak” isotherm consisting of Gibbs Surface excess,and the essential reason for the peak inflection point is that Gibbs discarded amount ρva has been reduced in the principle of isothermal adsorption,not negative ad- sorption,supercritical adsorption and other reasons. ② The critical pressure for the peak inflection point of Gibbs ad- sorption isotherm is determined by Gibbs discarded amount ρva ,in which the volume of adsorption phase va is deter- mined by coal structure,and the density of free phase ρ is determined by test temperature and pressure conditions. ③ Coal and shale adsorption belongs to physical adsorption,once the saturated adsorption pressure is reached,their actual adsorption isotherms will inevitably form a Langmuir-type linear platform for saturated adsorption. The intercept method deduces the adsorption phase density and the actual adsorption isotherm by the descending characteristics of Gibbs iso- therm,which,and therefore,conform to the physical characteristics of saturated adsorption and are reliable. The liquid phase method regards the liquid phase density as the adsorbed phase density,it not only lacks the theoretical basis,but also has a great error. Langmuir model method needs to set the initial and boundary values of unknown parameters in the process of fitting adsorption phase density with LM algorithm,and thus there is a large artificial error. ④ It is un- reasonable to calculate the adsorption constant and the unconventional resources by using the unadjusted adsorption isotherm measured in laboratory,because the unsaturated adsorption error and the neglect of Gibbs discarded amount are often caused by the lower test pressure using volumetric method,while the “peak” type adsorption isotherm meas- ured using gravimetric method under high pressure must be corrected before they can be used.