Seawater electrolysis is a promising route for hydrogen production using inexhaustible seawater resource. However, chloride ion (Cl⁻)-induced corrosion and limited catalytic activity pose challenges to the development of non-noble metal catalysts for seawater electrolysis in anion exchange membrane (AEM) electrolyzers. Herein, we fabricated an NiMo/NiMoP heterointerface to construct a strong built-in electric field (BEF) and further elucidated the impact of BEF on both electrocatalytic enhancement and corrosion resistance during seawater electrolysis. Theoretical simulations revealed that BEF benefitted the H adsorption, and simultaneously, the presence of an NiMoP layer effectively impeded Cl⁻ adsorption owing to steric effects. The NiMo/NiMoP heterointerface with a continuous BEF (NiMo/NiMoP-C) exhibited superior activity and durability compared with its counterpart with isolated BEF (NiMo/NiMoP-I). The excellent performance of NiMo/NiMoP-C in AEM seawater electrolysis was confirmed as it exhibited a current density of 1.0 A cm⁻² at a cell voltage of 1.8 V and a durability for over 350 h. This study provides a novel strategy for BEF construction to achieve a highly active and corrosion-proof non-noble metal catalyst for hydrogen production through AEM seawater electrolysis.