Lithium-sulfur batteries are promising next-generation energy storage devices due to their high energy density and low material cost. The efficient conversion of lithium polysulfides to lithium sulfide (during discharge) and to sulfur (during recharge) is a performance-determining factor for the lithium-sulfur batteries. Here we show that MoS_2-x/reduced graphene oxide (MoS_2-x/rGO) can be used to catalyze the polysulfide reactions to improve the battery performance. It was confirmed, through the microstructural characterizations of the materials, that sulfur deficiencies in the surface participated in the polysulfide reactions and significantly enhanced the polysulfide conversion kinetics. The fast conversion of soluble polysulfides decreased their accumulation in the sulfur cathode and their loss from the cathode by diffusion. Hence in the presence of a small amount of MoS_2-x/rGO (4 wt% of the cathode mass), the sulfur cathode improved its high rate (8 C) performance from a capacity of 161.1 mAh g^-1 to 826.5 mAh g^-1. In addition, MoS_2-x/rGO also raised the cycle stability of the sulfur cathode from a capacity fade rate of 0.373% per cycle (over 150 cycles) to 0.083% per cycle (over 600 cycles) at the typical 0.5 C rate. These results provide the direct experimental evidence for the catalytic role of MoS_2-x/rGO in promoting the polysulfide conversion kinetics in the sulfur cathode.