Unsupported MoS2 catalysts were synthesized by a one-step hydrothermal method using ammonium heptamolybdate and thiourea at different temperatures ranging from 150 degrees C to 180 degrees C. With the decrease of hydrothermal temperature, the obtained MoS2 catalyst showed an increased sulfur-resistant methanation performance. The physical structure and chemical characteristics of the catalysts were analyzed by N-2-physisorption, XRD, SEM, HRTEM, Elemental Analyzer, H-2-TPR, XPS and Raman techniques. Combining the catalytic performance trend with the characterization analysis results, we found that the catalysts obtained at lower temperature contain more bridging S-2(2-) groups, that is beneficial for H-2 dissociation and methanation reaction. We deduced that over-stoichiometric sulfur in the catalyst existed in the form of bridging S-2(2-) groups, which increased with the decrease of hydrothermal temperature. The finding that the positive role of the bridging S-2(2-) groups rather than the S vacancies in MoS2 catalyst is helpful for designing high efficient MoS2 catalysts not only for methanation but also for other hydrogenation reactions.