Abstract: The paper is a case study in liaoning,On the base of the routine weather data and the constant observational data of soil moisture for 18 representative stations during 1981~2002, the field actual evapotranspiration dynamic model of five regions in the Liaoning province was established under different supply water. By using the function relationship, the actual evapotranspiration was calculated in natural condition in lianning .The Results showed that the annual change of the actual evapotranspiration was in single-peak type,the peak value occurred during booting flower stage of corn.The regional distribution of the actual evapotranspiration was higher midst than all around,the trend of regional distribution of evapotranspiration didn’t conform to that of natural precipitation.The field actual evapotranspiration wave during 1981~2002, It is the higher the field actual evapotranspiration of the more precipitation year than that of the natural precipitation year, the actual evapotranspiration is appearing the increasing tendency in the central, eastern,southern and northern region of Liaoning province in 1990s,but the actual evapotranspiration is appearing the descending tendency in the western region of Liaoning province in 1990s.