Abstract: In order to study the measurement accuracy and error of absolute adsorption quantity of coal-bed methane in laboratory, a theoretical analysis and experimental methods were used. Based on the principle of determining the adsorption quantity of coal-bed methane in laboratory, the adsorption capacity that measured in the laboratory is actually the excess adsorption. On the other hand, the true adsorption quantity of coal-bed methane was called the absolute adsorption quantity. The excess adsorption quantity is less than the absolute quantity. By comparing the relationship between the excess adsorption capacity and the absolute adsorption capacity and the modified adsorption isotherm model, it is found that the adsorption isotherms are separated when the pressure becomes larger and the corresponding isotherm is lower than the latter. With the increase of pressure, the methane is close to the supercritical condition, the free phase methane density is bigger, and this trend becomes more obvious. At the same time, by analyzing the error of the excess adsorption quantity and the absolute adsorption quantity, the error between the absolute adsorption quantity and the excess adsorption quantity increases with increasing adsorption equilibrium pressure.