Tungsten disulfide (WS₂) is a promising and low-cost material for electrochemical hydrogen evolution reaction (HER) and has been extensively studied due to its excellent performance. However, the development of a facile and controllable defect-engineering to activate its basal planes is still crucial to improve its HER activity. Here, we put forward an annealing strategy to create controllable sulfur vacancies (S-vacancies) in ultrathin WS₂ nanosheets, which can result in the increase of active sites and enhanced electrocatalytic activity accordingly. Our density-functional-theory (DFT) calculations reveal that the Gibbs free energy of hydrogen adsorption (ΔG_H*) can be tuned to near zero by controlling the density of S-vacancies, leading to thermal-neutral HER performance. We find that optimal HER performance can be achieved by tuning the density of S-vacancies in WS₂ through annealing in the mixture of Ar and H₂ (5 %). The WS₂ nanosheets with the optimal density of S-vacancies show lower overpotential by 116 mV at 10 mA/cm² and smaller Tafel slope by 37.9 mV/dec than as-prepared counterpart, and super-excellent stability in acid. Additionally, the WS₂ with optimal S-vacancies also shows the best HER activity in alkaline solution. Our findings present a facile and general strategy to design electrocatalysts with more active sites, which is applicable to other materials for the improvement of their catalytic activities.