Lakes and reservoirs are important components of freshwater. The expansion and shrinkage of lakes/reservoirs may alter meteorological characteristics and the underlying surface conditions, which would further affect energy and water vapor fluxes in the surrounding area. In this study, the expansion and shrinkage of the Guanting Reservoir during 2013-2017 was analyzed using remote sensing data. Data collected from the Huailai Remote Sensing Experiment Station were used to analyze the energy and water vapor fluxes. The results showed the annual expansion of the Guanting Reservoir from 2013 to 2017, and a seasonal variation characterized by expansion in spring, shrinkage in summer and autumn, and expansion again in winter was exhibited. Meanwhile, the evapotranspiration (ET) in the surrounding area also increased annually. In the growing season, the seasonal shrinkage of the reservoir indirectly affected ET through net radiation, deep soil moisture and vegetation growth conditions, while in the non-growing season, the seasonal expansion directly increased ET by increasing the proportion of water bodies in the source area and increased net radiation and surface soil moisture. In addition, with the reservoir expanding year by year, the difference in ET between the closer site and further site from the reservoir increased obviously, especially in the non-growing season during the seasonal expansion of the reservoir. The results help with the ecosystem restoration and sustainable development of lakes/reservoirs in arid and semiarid areas.