Understanding the variation of active structure during the hydrogen evolution reaction (HER) process is of great importance for aiding in the design of optimized electrocatalysts. Herein, we present a composite material of FeP nanoparticles coated by N-doped carbon (FeP@NC) as an efficient HER electrocatalyst, synthesized by a pyrolysis and equivalent-volume impregnation method. The as-prepared FeP@NC catalyst can accelerate the HER at a small overpotential of 135 mV with a current density of 10 mA cm-2 in acidic medium and also shows a robust long-term stability with a minor decay of about 10% of the initial current density after 15 h. Using in situ X-ray absorption spectroscopy (XAS), a potential-dependent surface rearrangement of a surface pentahedral Fe structure into an octahedral Fe moiety via surface hydroxylation is clearly observed during the HER process, resulting in a much higher electrocatalytic activity. The theoretical calculations further unveil that the rearrangement of the surface FeP5(OH) octahedral structure could effectively trigger the adjacent P atoms to act as favorable proton acceptor sites towards improving the reaction kinetics of the Volmer step for efficient electrochemical hydrogen evolution.