The near-surface structure of the Pt-based alloy including the surface and subsurface structures is prominent to their electrocatalytic performance. Modulating the near-surface structure of PtCo intermetallics with small particle size could efficiently optimize the binding force between Pt and oxygen and finally enhance its oxygen reduction reaction (ORR) performance. Here we simultaneously achieve the size controlling and surface modulation of intermetallic nanoparticles (NPs) in a weak-oxidative confined space with abundant uncoordinated oxygen atoms. 1–2 atomic layers of concave Pt-rich surface were successfully constructed on 4 nm L1₀-PtCo core after removing Co–O species which is derived from the segregation of the subsurface Co to the surface induced by the uncoordinated oxygen atoms. Owing to the elaborate structure, PtCo-1000/C catalyst shows significant improvement in both activity (1.290 A∙mg_Pt⁻¹ and 1.529 mA∙cm_Pt⁻² at 0.9 V vs. reversible hydrogen electrode (RHE)) and stability (85.2% of initial mass activity after accelerated degression tests (ADTs)) even the production is scaled up to gram level. Density functional theory calculations suggest that the cave Pt site optimizes the protonation of *O, which finally boosts the ORR performance.