Abstract: The isothermal adsorption characteristics of shale gas under high pressure are investigated with a magnetic suspension mass method isothermal adsorption instrument. The experimental results showed that the adsorption characteristics of shale gas at low pressure conform to the Langmuir model. After the experimental pressure exceeds 10-12 MPa, the adsorption of shale shows obvious excess adsorption. Under high pressure, the sample container volume decreases in an exponential manner, which is related to the compressibility of the sample container under high pressure. The volume of shale samples increases exponentially and tends to equilibrate within a relatively small pressure range, which is related to the slight adsorption of He by shales. The densities of adsorbed phase CH₄, acquired from the linear fitting with the excess adsorption capacities and the densities of gaseous phase CH₄, are also dynamic. The adsorbed phase CH₄ density is close to the true one when using the contiguous excess adsorption capacities and the densities of gaseous phase CH₄ after the maximum excess adsorption. Combined with the dynamic changes of sample container volume, sample volume and the adsorbed phase CH₄ volume before and after the maximum excess adsorption, the absolute adsorption capacities can be calculated, which can present the adsorption characteristics of shale gas accurately. The absolute adsorption of shale gas under high pressure also follows the Langmuir model.