The invention provides a method for accurately calculating the real adsorption quantity of methane on shale. The method comprises the following steps: crushing muddy shale samples; drying in a thermostat; obtaining the change data of quality of the muddy shale samples along the gas pressure, and calculating the initial volume and quality of samples and a sample cylinder; calculating the excessive adsorption quantity of the samples under different pressures; making a plot of the quality and gas density of measured samples at high-pressure section in a rectangular Cartesian coordinate system, and fitting to obtain an analytic expression, requiring fitting R<2>=1; calculating the saturated adsorption quality and saturated adsorption phase volume of the samples, wherein the ratio of the two is density value of an adsorption phase; and correcting the excessive adsorption quantity of saturated adsorption points on an absolute line through the adsorption phase volume, and correcting the excessive adsorption quantity on front other points through adsorption phase density, thus obtaining the true adsorption quantity of methane on shale.