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-MoO3-x (A-MoO3-x) and MoS2 by in situ oxidizing amorphization of S-vacancy MoS2. The optimal A-MoO3-x/MoS2 catalyst exhibited a competitive HER overpotential of -146 mV at η = -10 mA cm-2. DFT calculations indicate that A-MoO3-x can reduce the energy barriers of water dissociation and H2 formation, and the heterointerfaces can facilitate charge transfer.