Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions provides an intriguing pathway to convert N2 into NH3. However, significant kinetic barriers of the NRR at low temperatures in desirable aqueous electrolytes remain a grand challenge due to the inert N≡N bond of the N2 molecule. Herein, we propose a unique strategy for in situ oxygen vacancy construction to address the significant trade‐off between N2 adsorption and NH3 desorption by building a hollow shell structured Fe3C/Fe3O4 heterojunction coated with carbon frameworks (Fe3C/Fe3O4@C). In the heterostructure, the Fe3C triggers the oxygen vacancies of the Fe3O4 component, which are likely active sites for the NRR. The design could optimize the adsorption strength of the N2 and NxHy intermediates, thus boosting the catalytic activity for the NRR. This work highlights the significance of the interaction between defect and interface engineering for regulating electrocatalytic properties of heterostructured catalysts for the challenging NRR. It could motivate an in‐depth exploration to advance N2 reduction to ammonia. We report an effective and feasible strategy to engineer oxygen vacancies in situ by constructing a heterojunction of Fe3C/Fe3O4 that enables an “optimized activation‐desorption” system for the electrochemical conversion of N2 into NH3. Combined with experimental results and DFT calculations, the synergistic effect between oxygen vacancies and heterojunction was further elucidated.