Abstract: Field ecosystem is an important part of terrestrial ecosystem, a study on H2O and CO2 flux change characteristics has significance to exploring carbon and water cycle in the whole terrestrial ecosystem. The corn farmland H2O and CO2 flux was measured under sunshine condition in heading period by the method of eddy covariance system, and the diurnal change characteristics were analyzed. The results showed that: (1) CO2 flux in farmland ecosystem increased with the increase of photosynthetically active radiation and the Michaelis-Menten function simulation reached extremely significant level, and the largest potential carbon flux was 3.63398 mg/(m2 · s); (2) CO2 flux had no obvious difference between morning and afternoon, and stomatal inhibition was not existent; (3) water vapor flux increased with the increase of photosynthetically active radiation, but water vapor flux in the afternoon was significantly higher than that in the morning, saturation vapor pressure of the afternoon was much higher than that of the morning, which caused much stronger transpiration and evaporation in the afternoon; (4) canopy conductance showed a reducing trend with time increase, which could reach a balance at 11:00, and reduced gradually at 14:00 with the decrease of photosynthetically active radiation, the reason could be the overestimate of canopy conductance in the morning caused by the evaporation of dew.