Sodium borohydride (NaBH4) is a promising solid-state hydrogen storage material because of its low toxicity, high environmental stability and release of high-purity hydrogen. Nevertheless, the practical application of NaBH4 is still hampered by its high desorption temperature and slow hydrogen exchange kinetics. Using experimental and first-principles calculations approaches, the dehydrogenation properties and modifying mechanisms of NaBH4+10 wt%graphene composite acquired by ball-milling are systematically investigated in this work. The results show that the graphene plays a cooperative catalysis-confinement effect on NaBH4. X-ray diffraction analysis displays that no new phases formed due to the mutual inertia between NaBH4 and graphene during ball-milling. Scanning electron microscopy and transmission electron microscopy observations show that the NaBH4 particles are significantly refined after graphene addition, which effectively restrains the agglomeration of NaBH4 particles. Thermogravimetry testing and mass spectrometry testing indicate that the onset dehydrogenation temperature of the NaBH4+10 wt%graphene composite is decreased by about 114 degrees C relative to the milled pristine NaBH4. First-principles calculations reveal that the enhanced dehydrogenation properties of NaBH4 after graphene addition should be ascribed to the reduced dehydrogenation enthalpy of NaBH4 and strong binding energy between NaBH4 and graphene as well as the electron transfer from NaBH4 to graphene.