The electrochemical ammonia synthesis is an environmentally friendly method for ammonia production; however, it is still impeded by the bottleneck of N₂ activation and challenging hydrogen evolution reaction (HER). A rational design for developing efficient electrocatalysts by tuning the surface electronic state is regarded as a promising strategy to overcome these obstacles. Herein, we report a phosphorus-doped potassium peroxyniobate (KNb₃O₈, denoted as P-KNO) electrocatalyst with enriched oxygen vacancies to effectively improve N₂-to-NH₃ efficiency and suppress HER. Such P-KNO electrocatalyst achieves a rate of 23.01 μg h⁻¹ mg_cat⁻¹ for NH₃ production at −0.45 V vs. reversible hydrogen electrode (RHE) and a faradaic efficiency (FE) of 39.77% at −0.4 V_RHE in a 0.1 M Na₂SO₄ electrolyte, which is about twice that of the non-modified KNb₃O₈ (denoted as KNO, 11.35 μg h⁻¹ mg_cat⁻¹, 19.60%) counterpart under the same condition. Moreover, the resultant P-KNO electrocatalyst renders steady NH₃ yield amounts and selectivity in cycling tests for 10 h. It is concluded that the enhanced N₂ fixation activity and faradaic efficiency are ascribed to the phosphorus doping and formation of oxygen vacancies, which not only make the surface of the electrocatalyst highly hydrophobic but also adjust the surface electronic structure of potassium niobate, thus resulting in accelerated N₂ adsorption and activation during the NRR process. This electrocatalyst with phosphorus-doping and oxygen vacancies gives deep insights into the rational regulation of the surface electronic state of NRR electrocatalysts for efficient NH₃ synthesis.