Using a quartz crystal microbalance system, the sorbed gas mass on tetrabutylammonium bis(trifluoromethanesulfonyl)imide [N-4 4 4 4] [NTf2] ionic liquid film coated on the surface of a quartz crystal under pressures from 0.1 to 4 MPa at 303 K was calculated based on the Sauerbrey equation. The experimental results are qualitatively close to the sorption simulation results, showing the rationality of experimental data. Although the amount of the sorbed gas on [N-4 4 4 4] [NTf2] ionic liquid film increases with pressure, CO2/CH4 selectivity only presents a slight fluctuation from 1.13 to 1.53, which is much lower than that in most fluid ionic liquid bulks possibly due to the dominant adsorption regime.