Abstract: This paper presents the comparisons of methane compressibility factors from universal three equations of state (EOS) and one empirical EOS. By the four EOSs the shale’s adsorption methane isotherms are calculated, respectively, and then the comparative analysis with the result given by the NIST standard reference database is conducted. The results show that adsorption isotherms calculated by different EOSs have significant differences and the EOS has less effect on the adsorption at equilibrium pressure less than 4 MPa, and the effect will quickly increase after mid?high pressure. Especially when the pressure is higher than 13 MPa, the shape of the isotherms will greatly change.It is mainly caused by gradually larger adsorption from different EOSs after the mid?high pressure, while at low pressure this effect is not big