This study presents a novel method for fabricating ultra-high-density single-atom catalysts (SACs) through pressure-controlled metal diffusion during pyrolysis. By reducing the pressure during the pyrolysis process, the aggregation of metal atoms is significantly inhibited, leading to a threefold increase in single-atom loading compared to ambient pressure. Molecular dynamics and computational fluid dynamics simulations reveal that reduced pressure enhances metal diffusion by increasing the mean free path of atoms, thereby minimizing agglomeration. The investigation of active site density through electrocatalytic oxygen reduction validates the robustness of this approach. The study demonstrates the first realization of Ullmann-type carbon–oxygen couplings catalyzed on single Cu sites, highlighting the potential for efficient heterogeneous catalysis. The method involves using nitrogen-doped graphitic carbon (NC) as a support and low pressures during pyrolysis. A wide range of SACs, including both nonprecious and precious metals, with single-atom loadings nearly three times higher than those obtained at ambient pressure, has been achieved. The SACs exhibit high performance in electrocatalytic oxygen reduction reactions (ORR) and high yields in Ullmann C–O coupling. The study also shows that pressure control significantly influences the diffusion behavior of metal atoms, with lower pressures promoting a hopping mechanism that enhances metal diffusion and distribution on the support. The results demonstrate that the pressure-controlled metal diffusion approach leads to the synthesis of SACs with high catalytic activity and stability. The Fe SAC and Cu SAC exhibit excellent performance in ORR and C–O coupling reactions, respectively. The synthesis of a series of SACs, including both nonprecious and precious metals, demonstrates the robustness of the method. This work represents a critical step toward scalable synthesis and widespread application of dense SACs, paving the way for the development of more economically feasible catalyst systems.This study presents a novel method for fabricating ultra-high-density single-atom catalysts (SACs) through pressure-controlled metal diffusion during pyrolysis. By reducing the pressure during the pyrolysis process, the aggregation of metal atoms is significantly inhibited, leading to a threefold increase in single-atom loading compared to ambient pressure. Molecular dynamics and computational fluid dynamics simulations reveal that reduced pressure enhances metal diffusion by increasing the mean free path of atoms, thereby minimizing agglomeration. The investigation of active site density through electrocatalytic oxygen reduction validates the robustness of this approach. The study demonstrates the first realization of Ullmann-type carbon–oxygen couplings catalyzed on single Cu sites, highlighting the potential for efficient heterogeneous catalysis. The method involves using nitrogen-doped graphitic carbon (NC) as a support and low pressures during pyrolysis. A wide range of SACs, including both nonprecious and precious metals, with single-atom loadings nearly three times higher than those obtained at ambient pressure, has been achieved. The SACs exhibit high performance in electrocatalytic oxygen reduction reactions (ORR) and high yields in Ullmann C–O coupling. The study also shows that pressure control significantly influences the diffusion behavior of metal atoms, with lower pressures promoting a hopping mechanism that enhances metal diffusion and distribution on the support. The results demonstrate that the pressure-controlled metal diffusion approach leads to the synthesis of SACs with high catalytic activity and stability. The Fe SAC and Cu SAC exhibit excellent performance in ORR and C–O coupling reactions, respectively. The synthesis of a series of SACs, including both nonprecious and precious metals, demonstrates the robustness of the method. This work represents a critical step toward scalable synthesis and widespread application of dense SACs, paving the way for the development of more economically feasible catalyst systems.