Abstract: Continuously measuring the evapotranspiration during the whole summer maize growing period in the Northern Plain，daily and seasonly variations of field evapotranspiration and their influence factors are analyzed．The results show that the total evapotranspiration of summer maize presents an obvious daily variation and the rate of evapotranspiration of summer maize varies with the growing periods．The net radiation，soil water and leaf area index can all affect the variations of the total daily evapotranspiration．Finally，the mathematic equation of summer maize to simulate the movement and transferring of soil water has been established．