Cost-effective and durable electrocatalysts for the alkaline hydrogen evolution reaction (HER) are urgently required. The slow HER kinetics suppressed by water dissociation hinder the application of catalysts in alkaline media. Herein, we constructed an amorphous heterostructure that combined amorphous-MoO_3−x (A-MoO_3−x) and MoS₂ by in situ oxidizing amorphization of S-vacancy MoS₂. The optimal A-MoO_3−x/MoS₂ catalyst exhibited a competitive HER overpotential of −146 mV at η = −10 mA cm⁻². DFT calculations indicate that A-MoO_3−x can reduce the energy barriers of water dissociation and H₂ formation, and the heterointerfaces can facilitate charge transfer.