Transition metal chalcogenides will be in situ transformed into metal oxyhydroxides during oxygen evo-lution reaction (OER) process in alkaline medium.However,most of these compounds only undergo sur-face reconstruction under operating conditions,which contains a large percentage of inactive atoms in the core,thus limiting the exposure of the active sites.Here,we synthesize a Ni-Mo-Se precatalyst with three-dimensional hierarchical structure and develop a facile on-site electrochemical activation strategy for achieving deep reconstruction of the precatalyst.Using the combination of multiple spectroscopic characterizations and high resolution electron microscopy techniques,we unravel that the Ni-Mo-Se pre-catalyst is deeply reconstructed into γ-NiOOH with co-leaching of Mo and Se after the anodic oxidation.Such flower-like γ-NiOOH is constituted by distorted ultrathin nanosheets with a thickness of ～ 4.5 nm and contains abundant intercalated species such as water and OH-/CO32-,thus offering a large quantity of accessible active sites.To reach the current density of 10 mAcm-2,the derived electrode requires an overpotential of only 244 mV,outperforming almost all the reported analogues.This work highlights the reconstruction chemistry and provides a simple method for the preparation of efficient OER electrocatalyst.