Developing Ru-based catalysts with high activity and long-term stability is of great significance for oxygen evolution reaction (OER) in acidic media. Here, a novel and rational strategy for the fabrication of ultra-small quasi-core/shell Ru-RuO2 nanostructures (∼4 nm) through graphdiyne (GDY) in-situ thermal reduction is reported. The as-prepared GDY supported ultra-small quasi-core/shell Ru-RuO2 (RuO2-Ru/GDY) exhibits an excellent OER performance in 0.5 M H2SO4 electrolyte, with an extremely low overpotential of 163 mV to attain 10 mA cm−2 as well as an over 75 h stability without significant increase in overpotential at 10 mA cm−2. The enhanced activity and stability of the catalyst can be attributed to the interaction between Ru core and RuO2 shell, where the former can effectively regulate the electron density of Ru in RuO2 shell, resulting in the optimal binding energy between Ru active sites and the oxygen intermediate for boosting the acidic OER ultimately.