Abstract: The instantaneous CO₂ flux concentration gradient 0.5m and 2.0 m higher above the canopy was measured with an infrared CO₂ analysis system during the growing season of summer corn. The field microclimatic factors such as radiation, temperature and moisture were measured simultaneously using the Bowen ration device, and the CO₂ flux density, latent heat flux and water use efficiency were also calculated. The results showed that the CO₂ analysis system was effective as the CO₂ flux density had the same variation trend as dry matter, and it proved useful as an advanced method to calculate the water use efficiency. The relationship between net photosynthesis rate and global radiation can be expressed as parabolic equation. When radiation reaches the light saturation point, the net photosynthesis rate will not increase any more. The light saturation point was affected by soil water content, and the radiation energy can make productive effect only with the affluent soil water. There was negative correlation between group water use efficiency and the relative water content in 0～60 cm soil. When the relative water content was in the range of 30.3%～80%, the water use efficiency decreased with the increase of soil water content. Some measures can be taken to develop the root to absorb the deep soil water and can increase the water use efficiency while the water supply did not increase.