[Objective] The evapotranspiration pattern, evolvement characteristics and water demand laws of ecosystem in typical agro-pastoral areas were studied to provide scientific basis for regional ecological management and water resources management.[Methods] Yanchi County of Ningxia Hui Autonomous Region was taken as an example, four-phase of Landsat remote sensing data and meteorological data from 2000 to 2017 were selected, and the daily evapotranspiration in different periods of the county was retrieved by using the SEBAL model, and the land use type data was analyzed and studied in the same period.[Results] ① The daily evapotranspiration at the beginning of the fall increased from 0.89 mm/d in 2000 to 1.71 mm/d in 2017, with an increase of 92.1%, designating that it had an increasing trend. ② There was a strong spatial heterogeneity in the daily evapotranspiration, and it was generally high in the south and low in the north, especially in the southeast loess hilly region, it was the highest. The annual growth rate of the past 17 years' evapotranspiration showed a characteristic of high in the south and low in the north, but there were large differences in the spatial pattern of annual increase in different periods. ③ There were differences in the evapotranspiration of different types of land. The average evapotranspirations of arable land, woodland and grassland were 1.42, 1.33, and 1.27 mm/d, respectively, but the largest annual increase in evapotranspiration was grassland. ④ The total amount of ecological water demand in Yanchi County and the amount of ecological water demand in various areas were increasing. The order of ecological water demand from high to low is grassland, arable land and woodland. However, the per unit ecological water requirement was the highest for arable land, and the smallest was of grassland. The shares of ecological water requirements of arable land and forest land in the total ecological water needs in the past 17 years was declining, while the proportion of grassland ecological water needs was on the rise.[Conclusion] Under the background of the intensive ecological management project in Yanchi County, the regional evapotranspiration increased significantly, the water demand of vegetation ecosystem increased obviously, and the evapotranspiration and water demand structure of different areas also changed.