The atmospheric carbon dioxide (CO₂) is the most important greenhouse gas in the global warming. Many researches concerning the CO₂ concentration enrichments impact on crop growth and product had been carried out based on the change of CO₂ concentration which observed from background station instead of from farmland station, yet the crop grew in farmland near to the village and town. In addition, few study paid attention to the variation of atmospheric CO₂ concentration over the farmland despite many researches focused on carbon cycle and carbon exchange. In order to know the long-term dynamics, trends and increment distributions of CO₂ concentration in farmland, we collected the data of CO₂ concentration observation of open pass eddy covariance system during 2007-2018 over the farmland in Gucheng Ecological and Agrometeorological Experimental Station of China Meteorological Administration. After data quality control and statistical analysis, we studied the dynamic characteristics of CO₂ concentration in farmland of North China Plain, such as inter-annual, intra-annual, diurnal and CO₂ flux, and compared the differences of CO₂ concentration and the trends in farmland of North China Plain with those in urban station and atmospheric background station. The results showed that the annual average concentration of CO₂ in the farmland of North China Plain had increased significantly from 361 to 392 μmol/mol in past twelve years (r=0.263, P < 0.01), with the average annual increment of 2.58 μmol/mol of CO₂ concentration, which close to that in global and Waliguan background station of China. However, the inter-annual and intra-annual variations of CO₂ concentration in the farmland fluctuated greatly with the standard deviation (SD) of 33.7 and 33.5 μmol/mol respectively. The average concentration of CO₂ in the night was 395.8 μmol/mol, higher 36.2 μmol/mol (10.1%) than that in the day in all the months, with the highest difference of 74.4 μmol/mol (20.6%) in August. In crop growth season, the two valley values of diurnal CO₂ concentration accurately matched the two peaks of the dynamic change of the CO₂ flux in May and August to September, which indicate that the dynamic changes of daily average CO₂ concentration and flux in day and night from April-September well reflected the relationships between the growth processes of winter wheat and summer maize, agricultural activities and carbon exchanges of farmland in the North China Plain. The dynamic change of CO₂ concentration in farmland was different from that in cities, wetlands and atmospheric background stations, suggesting the different formation mechanisms of CO₂ dynamic change. The characteristics of diurnal and night and seasonal variation of CO₂ concentration in farmland can provide guidance for studying and evaluating the effects of elevated CO₂ concentration on crop growth and yield.