Sodium hydride (NaH) was unprecedently embedded inside graphene nanobubbles via the discovered reaction between NaH and CO. With the graphene nanobubble as a nanoreactor for NaH, we directly observed the electron-beam-induced decomposition process of graphene-covered NaH by in situ high-resolution transmission electron microscopy with energy dispersive spectrometry and electron energy loss spectroscopy, revealing its decomposition mechanism. This can provide guidance for the design of hydrogen storage materials.