MoS2 is well-known for its catalytic properties, mainly to adsorb hydrogenous or carbonaceous materials. However, the effect of MoS2 on the oxygen adsorption has been investigated only a few times thus far. In this work, we first studied the adsorbability of O-2 by MoS2 through the analysis of Li2O2 growth on the surface of flower-like MoS2 microspheres with different concentrations of sulfur vacancies, which can be applied as the highly active electrocatalysts for Li-O-2 batteries. The enhancement of battery performance for the Def-MoS2@CTs (CTs = carbon textile substrates) with a larger concentration of sulfur vacancies (S/Mo = 1.61) can be achieved. The experimental and theoretical results confirm that the sulfur vacancies play a crucial role in the adsorption process and thus affect the morphology and nucleation of Li2O2. In addition, a fundamental catalytic mechanism for this adsorption process is also proposed. These results provide a new insight into the development of a highly active electrocatalyst by introducing a large concentration of defects for Li-O-2 batteries.