In this paper, hydrate phase equilibrium data for the CO2/CH4 + water system, CO2/CH4 + tetrabutylammonium bromide (TBAB) + water system, CO2/CH4 + tetrabutylammonium chloride (TBAC) + water system, and CO2/CH4 + tetrabutylammonium fluoride (TBAF) + water system were measured at temperatures from 280.2 K to 291.3 K and pressures from 0.61 MPa to 9.45 MPa with the 2.93.10(-3) mole fraction of tetrabutylammonium halide. The equilibrium hydrate formation conditions were measured by an isochoric pressure-search method. The mole fractions of the mixture gas used in this work were 0.33 CO2 and 0.67 CH4. The experimental data for the CH4 + water system were contrasted with the published equilibrium data in the literature. Both have a good consistency, which demonstrates that the experimental method and the apparatus used in this paper are feasible and reliable. The experiment results show that the hydrate stable region was enlarged by adding TBAB, TBAC, or TBAF. Among the three additives, TBAF is the best and the enlarged extent order of three additives is TBAF > TBAC > TBAB. The three-phase equilibrium pressure of the CO2/CH4 + TBAF + water system is 0.61 MPa at 284.2 K.