Although there is a general consensus that the electrocatalysts will undergo reconstruction to generate (oxy)hydroxides as real active sites during the electrochemical oxygen evolution reaction (OER), the understanding of this process is still far from satisfactory. In particular, the reconstruction process of most of these electrocatalysts is either slow or occurs only on the surface, which thus restrains the OER performance of the electrocatalysts. Herein, we reveal a fast and deep reconstruction of the coprecipitated Fe phosphates on nickel foam, via in situ Raman spectroscopy together with electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests. The generated NiFe (oxy)hydroxide nanosheets after reconstruction behave as the real active sites for the OER in the alkaline condition, with a low overpotential and excellent durability. The present work provides deep insights on the reconstruction dynamics of OER electrocatalysts.