Single-atom catalysts (SACs) with M-N₄ structure have drawn significant attention due to the facile preparation, maximum atom efficiency, unique electronic properties, uniform active sites, and excellent activity. Such catalysts integrated the merits of traditional homogeneous and heterogeneous catalysts effectively solve the cost, activity, and reuse problems. More importantly, the M-N₄ structure is flexible and other species like atoms, groups, and particles can be added to precisely control the local environment of M-N₄ to further improve the catalytic performance. Although unprecedented progress has been made, it remains difficulties in the rational design and controllable synthesis of a suitable SAC for a certain application. This review introduces the progress of M-N₄ catalysts and summarizes the strategies to modulate the M-N₄ structure, including changing the coordination number, tailoring the coordination structure, coordinating with groups, creating dual-atom catalysts (DACs), and coexisting of SAC with DAC and cluster. Special emphasis is placed on the preparation, structure characterization, and reaction mechanism of M-N₄-derived catalysts. Finally, the current challenges of these catalysts are also discussed to provide guidelines for the future design of efficient catalysts.