From May to October of 2004,based on the measurements of the diurnal and monthly transpiration rate,the evapotranspiration of a 17-year old Picea asperata Mast.and Larix gmeini Rupr mixed stand can be calculated through the transpiration area of trees in Datong County Qinghai Province.The water consumption of trees in the growing seasons can be reckoned through the evapotranspiration of plants,and the forestland evapotranspiration and water balance can be computed.The main results are as follows: Firstly,the total evapotranspiration of Picea asperata Mast.and Larix gmeini Rupr mixed stand in the growing seasons was 251.80 mm,the evapotranspiration of trees to the total field evapotranspiration and rainfall at the same period accounted for 80% and 58%.Secondly,the evapotranspiration of shrub and herb to the total field evapotranspiration accounted for 13%.Thirdly,the soil evaporation was 25.71 mm,accounting for 7% of the total evapotranspiration.