Metal-support interaction (MSI) has profound impacts on the catalytic performance of heterogeneous catalysts. Rational modulation of MSI will give rise to unusually high activity and stability. Here, we demonstrate that the MSI strength can be effectively tuned by the tunnel size of MnO2 supports to help address the two fundamental challenges in Ru-based acidic oxygen evolution reaction (OER): the sluggish kinetics and the instability of Ru sites. Through crystallographic engineering from α-MnO2 to β-MnO2 polymorphs, we found that the reduced tunnel size of MnO2 increases planar oxygen (Opla) concentration and promotes the formation of strong Ru-Opla-Mn bonds, thereby enhancing the Ru/MnO2 interactions. However, an excessively small tunnel size in β-MnO2 leads to surface amorphization and elongated Ru-Opla-Mn bonds after Ru incorporation, thus reversely weakening the Ru/MnO2 interactions. Our work manifests distinct volcano-shaped dependencies for both MSI strength and OER activity as a function of the tunnel size of MnO2 supports. The optimized Ru-γ-MnO2 catalyst, featuring an intermediate tunnel size and the strongest MSI, achieves an exceptional mass activity (1743 A g-1 at 1.5 V) while maintaining a high stability. Our results suggest that strong Ru-Opla-Mn interactions promote the formation of the OOH* intermediate through high Ru-O covalency and stabilize reactive Ru species against dissolution through double-exchange charge transfer from low-valence Mn sites. These findings offer valuable insights into the modulation of MSI via structural design of support for the optimization of other supported catalysts.