ZnS is a promising sorbent in recovering Hg⁰ from industrial flue gas due to its excellent Hg⁰ adsorption capacity. However, the internal structure–activity relationship still needs to be further clarified. In this work, ZnS sorbents with different structures were synthesized with the hydrothermal method by tuning the temperature. The samples had significant differences in the crystallinity, morphology, particle size, and sulfur (S) active sites. The results indicated that Hg⁰ removal performance was determined by the specific surface area and S active sites. ZnS synthesized at low temperatures (80-ZnS and 120-ZnS) had a larger surface area, while the S sites on the high-temperature-synthesized sample (160-ZnS) were more active for Hg⁰ adsorption. The 160-ZnS sample exhibited a much higher Hg⁰ adsorption amount per unit surface area. Further characterization revealed that S₂^2– and S_x were the main active sites for Hg⁰ adsorption. S_x existed in the form of long-chain polysulfur (L-S_x) on 80-ZnS and 120-ZnS, while it exhibited in the form of short-chain polysulfur (S-S_x) on 160-ZnS. L-S_x had negligible adsorption ability, while S-S_x had a high affinity for Hg⁰. Hg⁰ can react with S₂^2– and S-S_x, forming α-HgS and β-HgS, respectively. The new insight in this work can provide theoretical guidance for the design and structure optimization of ZnS, facilitating its practical industrial application.