The separation of CO2 by clathrate hydrate formation was considered to be a promising and economically feasible technology compared with the traditional techniques. In this work, we investigated the effect of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4) on the separation of CO2 from a 33 mol % CO2/67.0 mol % CH4 gas mixture based on hydrate formation in 0.05 mass fraction tetra-n-butylammonium bromide (TBAB) aqueous solution. The influence of both the [BMIn]BF4 concentration and the operating conditions on the gas separation by hydrate formation was studied. The results showed that the content of methane in the residual gas increased and the time required to reach balance was significantly shortened when the two additives were used in combination. The highest CH4 concentration in the residual gas phase was 84.0%, obtained in TBAB solution with 1200 ppm [BMIm]BF4 at 278 K and 3 MPa by one-stage hydrate separation. The maximum CO2 separation factor of 10.3 was obtained in TBAB solution with 1200 ppm [BMIm]BF4 at 283 K and 3 MPa. Accordingly, the novel additive of [BMIm]BF4 in TBAB solution promotes hydrate formation and gas separation.