The geological conditions and principal characters of CO2 changes in Huailai county are described in brief, with the emphasis on gradually rising of the observation values year after year since 2000. Some possible influencing factors such as test pipe, underground temperature and rainfall are ruled out by research, and similar characteristic of CO2 changes in Songshan, Yanqing county, where is 40 km away from Huailai, show it is not result of artificial environment changes. Further study shows that characteristic of CO2 changes is consistent with the rising of magnitude of earthquake gradually in 300 km around, which probably indicating the process of energy-accumulation, stress-intensification.