Hydrogen is a promising clean energy carrier, and electrocatalytic water splitting via the hydrogen evolution reaction (HER) represents a sustainable production strategy. While platinum nanoparticles (PtNPs) typically exhibit superior HER performance, their immobilization requires meticulous consideration due to an inherent aggregation tendency in solution. In this work, precise size-control of PtNPs was achieved by employing pyrene-based covalent organic frameworks (COFs) as effective hosts. Two COFs with similar mesoporous skeletons (IL-COF-2 and Azo-COF) and their sulfur-rich derivatives (TIL-COF-2 and TAzo-COF) were synthesized. These COFs feature ordered one-dimensional channels with diameters between 2.4 and 2.9 nm. Benefiting from the synergistic effects of spatial confinement and heteroatomic interactions, ultrafine PtNPs were uniformly immobilized within these COFs, with average particles sizes ranging from 1.7 to 1.9 nm. The resulting Pt@COFs all demonstrated excellent catalytic activities across a universal pH range, including acidic (0.5 M H₂SO₄), alkaline (1 M KOH) and neutral (1 M PBS) media. Among them, the heteroatom-rich Pt@TAzo-COF exhibited the most outstanding HER performance. This study realizes the precise size-matching between PtNPs and COFs, providing a robust strategy for effective Pt immobilization and enhanced pH-universal HER efficiency.