Efficient transition metal sulfide electrocatalysts for alkaline hydrogen evolution reaction (HER) are desired but restricted by their sluggish kinetics. Herein, we report a hydrothermal sulfurization-acid assisted etching strategy for the controllable fabrication of Mo doped NiS/Ni3S2 polymorph heterostructure with rich sulfur vacancies (Mo-NiS/Ni3S2-rich Sv). Direct spectroscopic evidence, together with theoretical analysis, demonstrate that the S centers on the Ni3S2 side of the nickel sulfide polymorphs are identified as the H2-evolving sites, while the Ni sites on the Mo-NiS side are beneficial for cleaving the HO-H bond. Importantly, in situ Raman spectroscopy further reveals that the presence of rich Sv can expedite the evolution of H* to molecular H2, promoting the HER kinetics. As expected, the optimal Mo-NiS/Ni3S2-rich Sv electrocatalyst exhibits the outstanding HER activity and excellent durability in alkaline solution. Understanding the synergy of polymorph heterostructure and element defect is crucial for the rational design of high-performance HER electrocatalysts.