The P-T stability conditions of gas hydrate in different systems (i.e., solution, silica sand, and marine sediment) were studied using multi-step decomposition method with our experimental equipment. The effects of different ions with various concentrations and sediment grains on the P-T stability conditions of gas hydrate were investigated. The results show that different ions have different influences on the phase equilibrium of gas hydrate. However, the influence of ions is in a similar trend: the larger the concentration, the bigger the P-T curve shifts to the left. For the silica sand, the influence of pore capillarity of coarse particles (> 460 μm) can be negligible. The P-T curve measured in coarse silica is in agreement with that in pure water. However, the influence of pore capillarity of fine particles (< 35 μm) is significant. The maximum reduction value of temperature is 1.5 K for methane hydrate under stable state. The sediment from the South China Sea significantly affects the P-T stability conditions of methane hydrate, with an average reduction value of 1.9 K within the experimental conditions. This is mainly the result of both the pore water salinity and the pore capillarity of sediment. Because the pore water salinity is keeping diluted by the fresh water released from hydrate dissociation, the measured P-T stability points fall on different P-T curves with the decreasing salinity. The PT stability conditions of gas hydrate in different systems (ie, solution, silica sand, and marine sediment) were studied using multi-step decomposition method with our experimental equipment. The effects of different ions with various concentrations and sediment grains on the PT stability the results show that different ions have different influences on the phase equilibrium of gas hydrate. the influence of ions is in a similar trend: the larger the concentration, the bigger the PT curve shifts to the left The influence of pore capillarity of coarse particles (> 460 μm) can be negligible. The PT curve measured in coarse silica is in agreement with that in pure water. However, the influence of pore capillarity of fine particles ( <35 μm) is significant. The maximum reduction value of temperature is 1.5 K for methane hydrate under stable state. The sediment from the South China Sea cts the PT stability conditions of methane hydrate, with an average reduction value of 1.9 K within the experimental conditions. This is mainly the result of both the pore water salinity and the pore capillarity of sediment. fresh water released from hydrate dissociation, the measured PT stability points fall on different PT curves with the decreasing salinity.