The greenhouse effect exacerbated by the increase of carbon-containing gases is the important cause of the climate change. On-line CO2 concentration monitoring of terrestrial ecosystem and CO2 flux analyzing are crucial for knowing regional climate and improving environment. The open-path CO2 monitoring system based on tunable diode laser absorption spectroscopy technology was designed and used for CO2 concentration in-situ detection in wheat field at State Experimental Station of Agro-Ecosystem in Fengqiu, Chinese Academy of Sciences in April, 2010. The continuing monitoring results show that the variation of CO2 concentration has an obvious diurnal periodicity in cropland mainly influenced by vegetation photosynthesis, and the general characteristic of diurnal variation is that the concentration decreased in the daytime and increased at night. This system detected gas concentration with large range, high sensitivity, short response time, easy calibration without gas sampling. Thus, it’s an effective optical remote-sensing method for CO2 concentration continuing monitoring in cropland.