Abstract: This paper describes The experimental methods of gas desorption of igneous rocks in vacuum and gas-genetic simulation of rocks at high temperatures. The quantitative assay of gas from thermal simulation is made on chromatograph and the C-isotope compositions of CO2 is analyzed as well. The isotope fractionation rules of CO2 from thermal simulation and nature gas, and their relations of CO_2 abundance and 5 13C values are studied in Sanshui basin, Guangdong. It can be concluded that there are two genetic sources of CO_2, in nature gas of Sanshui basin. One is the inorganic genesis of the mixture from the crust and The maple and the other is the cracking of organic matters.