The hierarchically structured flower-like CaF2 microspheres with nanopetals, named FL-CaF2, were synthesized via a hydrothermal method using calcium acetate Ca(Ac)2 and NaBF4 as calcium and fluorine sources, respectively, assisted by the chelating reagent trisodium citrate (Na3Cit) with the optimal pH of the synthesis solution. Meanwhile, a reference sample, named FL-CaF2-R, was reproduced using ethylenediaminetetraacetic acid disodium salt (Na2EDTA) as the chelating reagent, based on the recipe and synthesis procedure from the literature. Various techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform-infrared spectroscopy, and N2 adsorption-desorption at 77 K were then used to characterize the synthesized samples. The results show that FL-CaF2 with a larger diameter has a much higher thermal stability than FL-CaF2-R because the larger the nanocrystallite size, the higher the thermal stability. The adsorption of water vapor on CaF2 is irreversible because CaF2 can interact with the adsorbed water molecules strongly. The dual-site Langmuir model was used to describe the measured adsorption isotherms of water vapor on FL-CaF2 at low water vapor pressures and 298, 308, and 318 K. FL-CaF2 has a much higher water-adsorption capacity than those reported in the literature. Furthermore, the isosteric heat of adsorption as a function of loading, derived from the measured isotherms, varies from ca. 46 to 43 kJ mol-1 in the whole loading range investigated. Finally, the applications of FL-CaF2 are anticipated in the dehydration of hydrogen fluoride gas as well as in catalysis.