Abstract: The northern continental slope of South China Sea is one of the most favorable areas for gas hydrate accumulation in this region. This paper deals with the control of water depth and heat flow over gas hydrate formation through the studies of the relationship among the bottom-simulating reflectors (BSRs), water depth and heat flow. Results show that the moderate heat flow area (70~83 mW/m²) is the most favorable for gas hydrate formation, and the water will become too deep if the heat flow increases. The cap rock is another factor, which plays important role. We discussed in this paper the lowest limit of water depth under different thicknesses of overlying cap rocks. The influence of cap rock thickness is obvious on the base of the gas hydrate stability zone and its thickness. With the water depth increases, the base of the gas hydrate stability zone (BGHSZ) becomes deeper and the thickness of the zone larger.