In the Loess Plateau,water is the main factor limiting plant growth,productivity and distribution of vegetation is strongly related to the water balance.This study studies the water balance of different ecosystems using evapotranspiration and climate variables(precipitation and temperature)by the linear regression and t-test.Results showed that(1)evapotranspiration experienced fluctuation but a not notably rise(p>0.05)in the past 15years(2000~2014)and precipitation is a key factor than temperature.Besides, evapotranspiration was greater in rainy area than dry area,higher temperature area than lower temperature area.The ratio of precipitation and evapotranspiration varies with the changing of precipitation. Areas with higher or lower ratio are decreasing,while areas with medium ratio are expanding.(2)Evapotranspiration of different vegetation types followed the descending order of forest,cropland and grassland, with grassland and cropland showing larger variation and vulnerability.(3)Water balance was a relative ratio of P and ETrather than absolute ratio.The evapotranspiration increases with precipitation in different ecosystem,but evapotranspiration continues to rise even precipitation has reached a certain threshold.