In this work, we measured the phase equilibrium data of trimethylene sulfide + methane hydrates in brine water systems with a NaCl mass fraction of 0.035, 0.070, 0.100, and 0.120 by using T-cycle methods. The dissociation enthalpies of trimethylene sulfidemethane hydrates were calculated by the ClausiusClapeyron equation based on the measured phase equilibrium data. The equilibrium pressure of trimethylene sulfide + methane hydrate is much lower than that of the cyclopentane + methane hydrate at the same NaCl concentration in the aqueous solution. Hence, trimethylene sulfide may be a promising water-insoluble chemical promoter. The phase equilibrium pressure of trimethylene sulfide + methane hydrate increases as the temperature rises at the same NaCl concentration. In addition, the concentration of NaCl has more remarkable influence on the equilibrium pressure at a higher temperature. The formation/dissociation enthalpies of trimethylene sulfide + methane hydrate decrease with the increases of the temperature and NaCl concentration, respectively.