Electrocatalytic nitrite reduction reaction (NO₂⁻RR) to synthesize ammonia (NH₃) has been constrained by sluggish kinetics of water dissociation and the weak adsorption of nitrite. In this work, we develop an in-situ reconstruction strategy that transforms Ni-doped BiO_2−x (NiBiO_2−x) to Bi/NiBiO_2−x, which exhibits excellent activity and selectivity for NO₂⁻RR to synthesize NH₃. Diverse ex-situ and in-situ characterizations reveal potential-driven structural transformation from NiBiO_2−x to Bi/NiBiO_2−x, which features dual Ni²⁺-Bi⁰ active sites. The Ni²⁺ site is able to reduce the water dissociation barrier from 0.79 to 0.41 eV, while concurrently the Bi⁰ site can strengthen NO₂⁻ adsorption to promote *NO₂H intermediate formation. Consequently, the in-situ constructed Bi/NiBiO_2−x catalyst with Ni²⁺-Bi⁰ catalytic pairs enable an excellent NO₂⁻RR performance, achieving a NH₃ Faradaic efficiency (FE_NH3) of 94.5% at −0.6 V vs. RHE. The present study opens the new direction to in-situ construct high-performance electroreduction catalysts for small molecule synthesis.