The natural attenuation rules of oil pollutants are investigated by carrying out a field experiment in a gas station in Beijing, using a set of soil gas sampling and monitoring device designed independently. The samples of soil gas were collected in the field and tested to examine the content of O2, CO2 and VOCs in the unsaturated zone. The results show that after 381 days' natural attenuation, the concentration of the contaminants at the polluted point deceased by 99.2%, while the ratio of BTEX to TVOC decreased from 17.0% to 12.1%. Moreover, the content of O2 decreased while that of CO2 grew as the soil depth increases at G3 point. Further theoretical analysis indicated that: (1) The test results of first-step experiment show that there was a certain level of contamination in G3 area. And after 381 days' natural attenuation, the content of the BTEX at G3 point fell below the environmental standard level, which means the control measures could be removed. (2) The origin of the contamination was proved as short-term sources, and no long-term sources of leakage existed. (3) An effective removal of contaminants in the soil was found under the influence of the natural attenuation. This method could be applied in long-term monitoring for similar contaminated sites. (4) The test of O2and CO2 contents was proved to be an effective approach to determine if the aerobic degradation happens.