Single-atom catalysts (SACs) have attracted widespread attention in the field of catalysis due to their unique atomic dispersion characteristics, extremely high atomic utilization efficiency, and excellent catalytic performance. Accurately identifying their microscopic chemical environment is of crucial importance for revealing the catalytic mechanism and promoting the innovative design of high-performance catalysts, especially the developing of machine learning (ML) driven precision chemistry. X-ray absorption fine structure (XAFS) technology has become the core means in SACs research because of its high sensitivity to specific elements, the ability to simultaneously detect electronic and geometric structures, and good sample compatibility. This review begins with the fundamental principles of XAFS technology, systematically elucidating its key applications and significant advantages in the characterization of metal sites in SACs and in situ detecting of catalytic reactions. Furthermore, from the perspective of integrating theoretical calculations with experimental studies, it delves into the importance of using XAFS in conjunction with first-principles calculations and other methods to accurately determine the fine structure of active sites. Finally, this review delivers an in-depth discussion of the key challenges inherent in XAFS characterization and presents a forward-looking perspective on its future development. This work aims to furnish a robust theoretical framework and practical guidelines to foster a profound understanding of the intrinsic relationship between the structural characteristics and performance of SACs, thereby facilitating the targeted design of SACs.