The pursuit of energy conservation and environmental protection has always been a hot topic in the catalytic fields, which is inseparable from the rational designing of efficient catalysts and an in-depth understanding of the catalytic reaction mechanism. In this work, fully-exposed Pt clusters were fabricated on the atomically dispersed Sn decorated nanodiamond/graphene (Sn-ND@G) hybrid support and employed for direct dehydrogenation (DDH) of ethylbenzene (EB) to styrene (ST). The detailed structural characterizations revealed the fully-exposed Pt clusters were stabilized on Sn-ND@G, assisted by the spatial separation of atomically dispersed Sn species. The as-prepared Pt/Sn-ND@G catalyst showed enhanced ST yield (136.2 mol_EB·mol_Pt⁻¹·h⁻¹ EB conversion rate and 99.7% ST selectivity) and robust long-term stability at 500 °C for the EB DDH reaction, compared with the traditional ND@G supported Pt nanoparticle catalyst (Pt/ND@G). The ST prefers to desorb from the fully-exposed Pt clusters, resulting in the enhanced DDH catalytic performance of the Pt/Sn-ND@G catalyst. The present work paves a new way for designing highly dispersed and stable supported metal catalysts for DDH reactions.