Although high-energy facets on metal oxides are usually active and preferred for gas sensing, it is difficult to expose them according to thermodynamics. In this work, nanocomposites of SnO2 and graphene are prepared by a hydrothermal method. The SnO2 nanoparticles change from a lance shape to an octahedral shape as the concentration of HCl in the solution is increased gradually from 6.5 to 10 vol %. However, the SnO2 nanoparticles have an elongated octahedral shape if the concentration of HCl is increased further. The octahedral SnO2 nanoparticles are mainly surrounded by high-surface-energy {221} facets, thus facilitating gas sensing. First-principles calculation shows that the surface energy and adsorption energy of the {221} facets are larger than those of the stable {110} facets, and so, the gas adsorption capacity on the {221} facets is better. Furthermore, because the Fermi level of the SnO2{221} facet is higher than that of graphene, the electrons are transferred from SnO2 nanoparticles to graphene sheets, enabling effective electron exchange between the composite and external NO2 gas. The excellent gas-sensing properties of the octahedral SnO2/graphene composites are ascribed to the high-surface-energy {221} facets exposed.