The prediction of plant water status is a key issue of water management. Water stress on crops may alter the energy balance at the soil-atmosphere interface and the change in canopy temperature, which in turn affects transpiration and photosynthesis. An experiment was conducted at the Yucheng Integrated Agricultural Experimental Station between 2003 and 2005 to study crop (wheat-corn) water stress index (CWSI) and its relationship with atmospheric evaporation demand and soil water constraint. Eddy covariance method was applied to measure CO₂ and water fluxes, as well as canopy temperature and soil water content. Diurnal change in CWSI was derived using half-hourly flux data. Results showed a linear relationship between CWSI and water use efficiency (WUE). Soil water was better correlated to average daily WUE than hourly WUE. We also found that differences between air and canopy temperature were closely related to soil water content.