The electrochemical nitrate reduction for green ammonia production is attracting increasing attention, where the catalysts are widely investigated by controlling the compositions or structures to achieve high performance. However, their reconstructions under reduction potentials are inevitable and uncontrollable, leading to uncertain performance, and a confused understanding of the mechanism. In this work, a strategy is proposed by controlling the pre-catalyst's reconstruction chemistry toward electrochemical nitrate reduction reaction (e-NO₃RR) with superior activity and stability. To demonstrate the idea, a pre-catalyst is fabricated with α-Co(OH)₂ and Cu(OH)₂ (α-Co(OH)₂/Cu(OH)₂), which is in situ reconstructed to a tandem catalyst with Cu and β-Co(OH)₂ (Cu/β-Co(OH)₂) under working potential. Cu/β-Co(OH)₂ achieves an optimal Faraday efficiency for ammonia of 97.7% with a yield rate of 3.9 mmol h^-1 cm^-2 at −0.5 V, outperforming other reported metal-hydroxide catalysts. The experimental and theoretical results demonstrate that a tandem catalytic mechanism is responsible for the exceptional performance: 1) Cu functions as the donor of nitrite; and 2) β-Co(OH)₂ serves as active sites for generating active hydrogen and reducing nitrogen-containing groups. This work highlights that the controllable reconstruction toward improved performance can be realized, and provides an insightful understanding of the mechanism, which is helpful for developing active and stable catalysts for various catalytic applications.