Abstract: To explain and solve the phenomenon that the negative value of gas adsorption appeared in the gas adsorption constant tests, the relative parameters of gas adsorption volume and the measurement error were analyzed, and the results show that the coal mass loss, empty tank volume error, true density error and high pressure adsorption equilibrium time are the main factors contributing to the negative value of gas adsorption. And the solving methods are proposed to provide reference for the accurately determination of gas adsorption constant.