Abstract: In order to meet the research work of oil and gas field and provide precise data for exploration and development, the carbon isotope value of gas samples of Ordos-basin were determined by GC-C-IRMS. The composition of gas in the field showed that the content of methane was high, while that of ethane, propane and heavier hydrocarbons were low. Therefore, the accurate carbon isotope value of ethane and propane was obtained by two injections. Through repeated experiments, it is found that the signal intensity of Almp 44 mV has a correlation with detection value. When the signal intensity of Almp 44 mV is less than 190 mV, the data stability becomes worse, and the number of samples is close to and beyond error range increased, when the signal intensity of Almp 44 mV is higher than 190 mV, the detection value tends to be stable. When in use, the carbon isotope value shall be combined with other geochemical parameter. As long as the instrument is stable, the environment temperature is constant, the data can be considered reliable, when Almp 44 mV signal strength is higher than 190 mV.