Electrochemical water splitting in acid has been emerging as a powerful, sustainable and green protocol to produce hydrogen gas sources. In this study, we propose a novel strategy to fabricate RuO_x clusters anchored on self-assembled SnO₂ cubic nanocages (RuO_x-SnO₂ composites), which is substantiated by a combination of spectroscopy and microscopy. The resulting RuO_x-SnO₂ composite catalysts exhibit boosting oxygen evolution reaction (OER) performance: A Tafel slope of 41.2 mV·dec⁻¹ and a low overpotential of 225 mV@10 mA·cm⁻² in a 0.5 M H₂SO₄ (pH=0) electrolyte are achieved, outperforming the state-of-art OER catalyst of commercial RuO₂ (com-RuO₂). Notably, RuO_x-SnO₂ gives an extraordinarily large mass activity of 6873.4 A·g_Ru⁻¹ at the overpotential of 270 mV, which is approximately 170 times higher than that of com-RuO₂ (40.2 A·g_Ru⁻¹). The RuO_x-SnO₂ exhibits a good durability for at least 100 h@50 mA·cm⁻² and > 500 h@10 mA·cm⁻² and a stability of 30 hours at 100 mA·cm⁻² in an assembled proton exchange membrane water electrolysis, indicating that the engineered microstructure possesses significant potential for practical applications. The high intrinsic OER performance is attributed to the increasing density of exposed catalytic sites by downsizing RuO_x clusters with abundant oxygen vacancies (Ov, 1.02×10⁻¹² spin·mg_cat.⁻¹ determined by electron paramagnetic resonance). Furthermore, a Ru5c-Ov dual-active site mechanism is proposed by density functional theory calculations, that is, the moderate surface migration between five-coordinated surface Ru site (Ru5c) and Ov makes the *O→*OOH rate-determining step feasible. Moreover, this strategy provides a novel route for enhancing acidic OER activity and highly encouraging for their future applications of ruthenium-based composite catalysts.