Abstract: Main working area of this paper is part of ocean from Qiongdongnan Basin to Xisha Trough in South China Sea. The data in this paper consisted: the data collected in the marine science investigation about gas hydrate in Qiongdongnan Basin and Xisha Trough from 2000 to 2006; the data from tested samples applied from ODP164、ODP204 and ODP308 where had found gas hydrate, which were tested for hydrocarbon compositions and acidolysis hydrocarbon isotopic compositions in laboratory; the data gathered from others papers. On the basis of analysis of distribution characteristics of methane, C1/(C2+C3) and characteristics of hydrocarbon isotopic compositions, we analyzed the cause of gas hydrate and the geochemical significance of acidolysis hydrocarbon isotopic compositions. Combined geochemical data with geological data, the best block for gas hydrate bearing was selected and the occurrence of gas hydrate in the working area was prospected.