The embodiment of the invention provides a method and a device for determining the adsorption quantity of a water-containing shale. The method comprises the following steps: obtaining a water-containing shale sample; performing an water-containing shale isothermal adsorption experiment on the water-containing shale sample to detect a first experimental equilibrium pressure, a first reference equilibrium pressure and a first communication adsorption equilibrium pressure; calculating the saturated water vapor pressure, and using the saturated water vapor pressure to respectively correct the above measured pressure data in order to obtain a second experimental equilibrium pressure, a second reference equilibrium pressure and a second communication adsorption equilibrium pressure; and determining the adsorption quantity of the water-containing shale according to the second experimental equilibrium pressure, the second reference equilibrium pressure and the second communication adsorption equilibrium pressure. The interference of water evaporation in the adsorption experiment is considered, the saturated water vapor pressure is used to correct the experimentally detected pressure data,and the corrected pressure data are used to determine the adsorption quantity of the water-containing shale, so the technical problems of poor accuracy and large error in the prior art are solved.