Electrochemical water splitting in alkaline media is an attractive way to produce the clear and renewable hydrogen fuel H2. In this work, we report a single-atom Fe1/NC catalyst, where the Fe-Nx moiety works as the active site, for high-efficiency alkaline hydrogen evolution reaction (HER). The Fe1/NC electrocatalyst exhibits a low overpotential of 111 mV at the current density of 10 mA cm-2, with a Tafel slope of 86.1 mV dec-1 in 1 M KOH solution. Operando X-ray absorption spectroscopy reveals that, under the working states, the Fe-support interaction weakened as the Fe-N coordination number and Fe oxidation state decreased. As such, the evolved single-atom Fe site with more d electrons provides a favorable structure for boosting HER performance. This work gives insight into the structural evolution of the active site under the alkaline HER and provides a strategy for the design of non-noble metal electrocatalysts.