Many researchers have estimated the thickness of the gas hydrate stability zone (GHSZ) and amount of gas hydrate at different areas in the South China Sea (SCS) using varying methods, but few have reported calculations for the whole of the SCS. In this paper, Milkov and Sassen’s model is used to calculate the thickness of the GHSZ in the SCS. In addition, an estimation of the amount of gas hydrate and discussed in terms of gas composition and the effects of water depth. Average thicknesses for the GHSZ in the SCS are estimated to be ~440 m, ~477 m and ~553 m based on the relationship between GHSZ thickness and water depth established in this study for 3 kinds of gas hydrates with gas composition containing 100% methane, 96% methane, and 90% methane, respectively. Then, by assuming that gas hydrates are distributed in half of the continental slope area of the SCS, and that the gas hydrate saturation is 1.2% of sediment volume, the amounts of gas hydrate are estimated to be ~3.2 × 1012 m3, ~3.4 × 1012 m3, ~4.0 × 1012 m3, and the volumes of hydrate-bound gases are ~5.2 × 1014 m3, ~5.6 × 1014 m3, ~6.5 × 1014 m3 for the 3 kinds of gas hydrates above, respectively. The results above show that water depth and gas composition are important factors affecting the thickness of the GHSZ in which the thickness of the GHSZ increases with water depth and decreases with the volume of methane, and that the gas hydrate may be a potential energy source in SCS.