Abstract: Based on analysis of water depth, seabed temperature and geothermal gradient etc. conditions in Deepwater Block Q of Qiongdongnan Basin, the authors calculated GHSZ (gas hydrate stable zone). It reveals two key points. Firstly, under the specific seabed temperature and geothermal gradient conditions in this block, the temperature and pressure requirements for gas hydrate formation can be met only in the sea area with water depth larger that 600 m; secondly, the thickness of GHSZ is proportionate with water depth, and the maximum thickness of GHSZ in this block is 350 m. Comparison of 3D seismic data from Block Q with the highresolution seismic data from Ulleung Basin of South Korea shows that they both have some similar abnormal seismic reflects such as abundant shallow gases, detrital flow accumulation bodies and gas chimney etc.. It is speculated that gas hydrate samples may be acquired from the abnormal reflects which indicate the gas hydrate. The bottom depth of GHSZ inferred from the T-P conditions in Qiongdongnan Basin can be used as constrain condition for the interpretation of shallow gas and gas hydrate.