Alkaline hydrogen evolution reaction (HER) is more sluggish than that in acidic media because of the additional energy required for water dissociation. Herein, we report a scalable strategy (alloying-dealloying) to synthesize self-assembled shell-core porous configuration as active and cost-effective electrocatalyst for alkaline HER. Benefiting from the dual function of oxygen vacancies (O_V) defects on NiMn₂O₄ shell and large specific surface area provided by Mn-doped nanoporous Ni core, the resultant O_V-NMO/np-Ni₃₀(Mn) electrode achieves a lower overpotential of 130 mV at 10 mA cm⁻², a higher exchange current density of 0.5011 mA cm⁻² and long-term operational durability in hostile basic environment. Density functional theory (DFT) calculations reveal that appropriate concentration of O_V in NiMn₂O₄ could decreases the energy barrier of Volmer step from 1.59 eV to 1.49 eV, suggesting O_V serves as a hot-spot for the dissociation of water molecules during alkali-HER. This work paves a novel approach to fabricate robust shell-core porous structure with abundant defects, dopants and active sites that are favorable for efficient and inexpensive alkaline hydrogen production.