Herein, we present the synthesis of oxygen vacancies-rich α-MoO3-x nanobelts through a novel defect-engineering strategy. The oxygen-vacancies could not only greatly increase the interlayer spacing and the electrical conductivity of MoO3, but also significantly enhance the electrochemical activity, which promotes faster charge storage kinetics. Meanwhile, to further facilitate the electron transfer and ion transport, a graphene nanomesh-carbon nanotube/MoO3-x (GC/MoO3-x ) nanocomposite with three-dimensional sandwiched structure was fabricated, which displays high specific capacity up to 306 C g−1 as well as high volumetric capacity of 692 C cm−3. Our fabricated asymmetric supercapacitor (ASC) with the GC/MoO3-x and GC/MnO2 nanocomposites as anode and cathode, respectively, exhibits an ultrahigh energy of 150 Wh kg−1, corresponding to an impressive volumetric energy density of 319 Wh L−1. Notably, both the gravimetric and volumetric energy densities are much higher than most of the previously reported metal oxide based ASCs in aqueous electrolytes. Furthermore, the ASC displays an ultra-long lifespan with 101% retention ratio after 30,000 cycles. The outstanding performances of GC/MoO3-x composite render it a highly promising candidate for next-generation supercapacitors with both high energy and power densities in future applications, especially in greatly limited space.