Surface evapotranspiration is a main factor influencing wetland water-heat balance and a main way of water loss. It has important significance for the reasonable determination of ecological water demand in wetlands and the effective management of water resources. With the help of remote sensing and GIS technology, the quantitative characterization of the phenological characteristics of the underlying surface of Zhalong wetland based on temporal NDVI data was made by using Landsat images of 34 scenes in 2002, 2010 and 2016. At the same time, the dynamic change information of land use/cover in three periods of the wetland reserve was obtained accurately. The SEBAL model with good physical foundation and wide application was selected to estimate the instantaneous evapotranspiration of the wetland. By combining with the meteorological data of nearby hydrographic stations, the temporal expansion of evapotranspiration of the wetland was realized. The daily, monthly and annual evapotranspiration of Zhalong wetland was obtained, and the spatio-temporal distribution characteristics of evapotranspiration in Zhalong wetland were deeply investigated. At last, the present ecological water demand of Zhalong wetland in three periods was quantitatively estimated from three aspects:water demand of wetland lake bubble, water demand of plants, and water demand of biological habitat. The results indicated that the land cover types of Zhalong wetland were mainly reed marshes, grasslands and cultivated land, among which reed marshes occupied the absolute dominance, increased by 205.82 km² continuously from 2002 to 2016. Grassland and cultivated land decreased by 119.35 km² and 95.96 km², respectively, indicating that Zhalong wetland ecosystem showed a recovery trend from 2002 to 2016. The annual evapotranspiration of the wetland reserve presented a roughly single-peak distribution in the three years, which was consistent with the summer > spring > autumn > winter. The annual evapotranspiration increased significantly from 518.87 mm in 2002 to 625.98 mm in 2016, by an increase of 20.64%. In order to meet the ecological consumption in the wetland, the total ecological water demand of the wetland increased correspondingly from 2002 to 2016. The range of the appropriate ecological water demand of the wetland reserve was 540 million m³ -708 million m³, which could maintain the health of the lakes, vegetation, animal and plant habitats in the wetland. The minimum ecological water demand for maintaining the health of the core area varied from 271 million m³ to 332 million m³. With the improvement of spatial and temporal resolution of remote sensing data, the estimation of wetland ecological water demand based on evapotranspiration inversion will be more practical and accurate, providing effectively technical support for the formulation of scientific and reasonable water rehydration schemes in wetland reserves.