β-NiOOH shows excellent oxygen evolution reaction (OER) catalytic activity under alkaline conditions, but the surface structure under the working environment is not yet fully established. Through ab initio thermodynamic approach, the relationship between catalyst surface structure and pH, electrode potential was studied. For the most stable state of the catalyst surfaces, three adsorbing oxygen atoms and one Ni vacancy form. Doped iron decreases the formation energy of Ni vacancy thus increasing the stable range of surface states. OER mechanism was explored based on the obtained most stable structures. The first step of the reaction is the desorption of oxygen rather than the formation of *OH due to large Steric effects. Ni vacancy reduces the dehydrogenation energy of the adsorbate *OOH and the formation energy of oxygen radical. The interaction between iron and oxygen atom is large than the Ni and oxygen. Therefore, Fe-doped system reduces the formation energy of *O and changes the potential-limited step from the formation of adsorbate *O to the desorption of O₂.