Tuning the local reaction environment is an important and challenging issue for determining electrochemical performances. Herein, we propose a strategy of intentionally engineering the local reaction environment to yield highly active catalysts. Taking Pt^δ− nanoparticles supported on oxygen vacancy enriched MgO nanosheets as a prototypical example, we have successfully created a local acid-like environment in the alkaline medium and achieve excellent hydrogen evolution reaction performances. The local acid-like environment is evidenced by operando Raman, synchrotron radiation infrared and X-ray absorption spectroscopy that observes a key H₃O⁺ intermediate emergence on the surface of MgO and accumulation around Pt^δ− sites during electrocatalysis. Further analysis confirms that the critical factors of the forming the local acid-like environment include: the oxygen vacancy enriched MgO facilitates H₂O dissociation to generate H₃O⁺ species; the F centers of MgO transfers its unpaired electrons to Pt, leading to the formation of electron-enriched Pt^δ− species; positively charged H₃O⁺ migrates to negatively charged Pt^δ− and accumulates around Pt^δ− nanoparticles due to the electrostatic attraction, thus creating a local acidic environment in the alkaline medium.