Effective and robust catalyst is the core of water splitting to produce hydrogen. Here, we report an anionic etching method to tailor the sulfur vacancy (V S ) of NiS 2 to further enhance the electrocatalytic performance for hydrogen evolution reaction (HER). With the V S concentration change from 2.4% to 8.5%, the H* adsorption strength on S sites changed and NiS 2 -V S 5.9% shows the most optimized H* adsorption for HER with an ultralow onset potential (68 mV) and has long-term stability for 100 h in 1 M KOH media. In situ attenuated-total-reflection Fourier transform infrared spectroscopy (ATR-FTIRS) measurements are usually used to monitor the adsorption of intermediates. The S- H* peak of the NiS 2 -V S 5.9% appears at a very low voltage, which is favorable for the HER in alkaline media. Density functional theory calculations also demonstrate the NiS 2 -V S 5.9% has the optimal |ΔG H* | of 0.17 eV. This work offers a simple and promising pathway to enhance catalytic activity via precise vacancies strategy.