The phase equilibrium data for semiclathrate hydrates of tetra-n-butylammonium chloride (TBAC) + methane with TBAC mass fractions (w) of 0.0500, 0.0997, 0.2000, and 0.3001 are measured in the temperature range of (281.65 to 292.85) K using a visual high-pressure phase equilibrium apparatus. The experimental data are generated using an isochoric step-heating method (T-cycle method). Methane partially occupied the small cages of TBAC semiclathrate hydrates, which increases the stability of the hydrates. The phase equilibrium pressure of TBAC + methane hydrates increases with the increase of temperature at a specified TBAC concentration. The experimental results present that the presence of TBAC decreases the formation pressure of methane hydrate greatly. Moreover, pressure reduction is dependent on the TBAC concentration. The experimental data also show that TBAC is a kind of promoter for methane hydrate formation.