Carbon dioxide (CO_2) is an important greenhouse gas and a recognized contributor to the global warming. An observation site equipped with an automatic CO_2 monitoring analyzer was set up at the Qianyanzhou Experiment Station of the Chinese Ecosystem Research Network, Chinese Academy of Sciences in Jiangxi Province, to monitor CO_2 concentration in the atmosphere. The monitoring data during the period from March 2013 to March 2014 and synchronous meteorological data were analyzed to explore background CO_2 concentration in the air of the region as representative of East China and its variation characteristics, as well as track of the air mass that influences the region. Results show that the mean CO_2 volume fraction concentration of the region was 409.2*10~(-6). CO_2 concentration varied sharply in a day, peaking at 07:00 and bottoming at 15:00 and also in a year with the seasons, being the highest in winter and the lowest in summer. Besides the impact of human activities, vegetation photosynthesis and altitude of the boundary layer were all main factors affecting CO_2 concentration in the atmosphere. Backward trajectory and PSCF (potential source contribution function) analysis reveals that CO_2 in the atmosphere of Qianyanzhou comes mainly from its northeast, the Yangtze River Delta region and its north, the cities in North China.