Single‐atom catalysts (SAs) with the maximum atom utilization and breakthrough activities toward hydrogen evolution reaction (HER) have attracted considerable research interests. Uncovering the nature of single‐atom metal centers under operating electrochemical condition is highly significant for improving their catalytic performance, yet is poorly understood in most studies. Herein, Pt single atoms anchoring on the nitrogen–carbon substrate (PtSA/N–C) as a model system are utilized to investigate the dynamic structure of Pt single‐atom centers during the HER process. Via in situ/operando synchrotron X‐ray absorption spectroscopy and X‐ray photoelectron spectroscopy, an intriguing structural reconstruction at atomic level is identified in the PtSA/N–C when it is subjected to the repetitive linear sweep voltammetry and cyclic voltammetry scanning. It demonstrates that the PtN bonding tends to be weakened under cathodic potentials, which induces some Pt single atoms to dynamically aggregate into forming small clusters during the HER reaction. More importantly, experimental evidence and/or indicator is offered to correlate the observed Tafel slope with the dynamic structure of Pt catalysts. This work provides an evident understanding of SAs under electrocatalytic process and offers informative insights into constructing efficient catalysts at atomic level for electrochemical water‐splitting system. The dynamic reconstruction from the isolated atoms to small clusters on the N‐doped carbon‐supported Pt single‐atom catalyst during the alkaline hydrogen evolution reaction (HER) is clearly revealed by in situ/operando spectroscopic characterizations, based on which a significant correlation between the Tafel slope for HER and the dynamic structure of Pt can be well established.