In this study, we investigated the deliquescence of NaBH4 thin films under atmospheric conditions toward their hydrogen supply application. Upon exposed to air, the film deliquesced along with generating hydrogen due to the decomposition of BH4- ions, but the decomposition rate was slow enough for a sizable amount of BH4- ions remained stable in the deliquesced film. As a result, a reversible deliquescence-recrystallization cycle in the NaBH4 thin film could be achieved through control of humidity. Molecular dynamics simulations suggested that Na+ and BH4- ions tended to neighbor each other with almost the same intra-distance as that in the crystal. A kind of such local ordered structures in deliquesced NaBH4 may prevent an excess hydration and subsequent decomposition of BH4- ions and is responsible for the stability of BH4- ions, enabling the reversible deliquescence-recrystallization cycle. These combined experimental and theoretical insights offer new perspectives on controlling hydrogen release from borohydrides based on humidity conditions in the thin film form. •Deliquescence of NaBH4 thin films on Si (100) substrates explored.•Raman spectra confirm prevalence of BH4- ions in deliquesced state.•In-situ XRD reveals reversible recrystallization and deliquescence cycle.•MD simulations suggest local molecular-like structure prevents excess hydration.•Insights gained into controlled hydrogen release, offering novel borohydride applications.