Based on the analysis of sea-bottom temperature and geothermal gradient, and by means of the phaseboundary curve of gas hydrate and the sea-bottom temperature versus water depth curve in the South China Sea, thispaper studies the temperature and pressure conditions for gas hydrate to keep stable. In a marine environment, methanehydrate keeps stable at water depths greater than 550 m in the South China Sea. Further, the thickness of the gas hydratestability zone in the South China Sea was calculated by using the phase boundary curve and temperature-depth equations.The result shows that gas hydrate have a better perspective in the southeast of the Dongsha Islands, the northeast of theXisha Islands and the north of the Nansha Islands for thicker stability zones.