Developing efficient cathode catalysts can largely promote the application of Li-O2 batteries (LOBs). In this work, the core-shell MoS2−x@CNTs composite is synthesized via a hydrothermal method with annealing and NaBH4 reduction post-processing, of which the defective MoS2 nanoflakes are homogeneously coated on the 3D carbon nanotube (CNT) webs. It is found that it delivers superior bifunctional catalytic activities toward both oxygen reduction and evolution reactions for LOBs. On the one hand, the charge re-distribution on MoS2 nanoflakes with sulfur vacancies can be effectively constructed by the surface engineering strategy, remarkably boosting the kinetics of Li-O2 catalysis. On the other hand, the conductive and high surface area CNT network can facilitate mass transfer and provide enough free space for composite cathodes, accommodating the volume changes caused by the reversible formation and decomposition of discharge products during cycling. More importantly, the unique core-shell architecture can not only enable fully covering of defective MoS2 nanoflakes on CNT surfaces to avoid the contact between CNTs and electrolyte, distinctly suppressing side reactions, but also realize the exposure of more active sites to fulfill their catalytic properties. This work provides an insightful investigation on advanced catalysts and holds great potential for catalyst structural engineering in LOBs.