<p>Electrochemical nitrate reduction to ammonia (NRA) has promising applications in sustainable wastewater treatment and ammonia production. However, its development is limited by the slow reaction kinetics and competition with the hydrogen evolution reaction (HER). Cu-based electrocatalysts have been extensively investigated for NRA because of their high electrochemical activity and tunable electronic structure, whereas monometallic Cu suffers from weak H^* adsorption properties. Herein, the highly dispersed Cu/WO₃ heterojunctions loaded on carbon fiber are successfully synthesized via the carbothermal shock reduction method, which can reach the ultra-high temperature in a short time to avoid the accumulation of the active site. WO₃ plays an important role in the generation of hydrogen radicals (H^*), which further promotes the continuous hydrogenation process from nitrate to ammonia. The optimal sample exhibits a significant NRA performance, achieving an ammonia yield rate of 158.66 µmol h⁻¹ cm⁻² and Faradaic efficiency of 98.27%. Electron paramagnetic resonance (EPR) and Density Functional Theory (DFT) calculations reveal that the Cu/WO₃ heterojunction provides synergistic effects at active sites. Specifically, Cu sites preferentially adsorb NO₃⁻, while WO₃ sites facilitate the dissociation of water to generate hydrogen radicals (H^*). This synergistic interaction between Cu and WO₃ enhances the overall catalytic activity for the NRA process. This work provides a new strategy for constructing heterojunction electrocatalysts to further develop NRA performance.</p>