Currently, the development of low-thickness, high-efficiency electromagnetic wave (EMW) absorbing materials for practical applications is of particular importance. In this work, we used a simple one-step hydrothermal method to synthesize, for the first time, heterostructured materials with special multidimensional heterostructures by exploiting the confinement effect of grapheneIn this work, a simple one-step hydrothermal method is employed to synthesize composites with a special heterogeneous structure through utilizing the confinement effect of graphene. NbS nanosheets are grown directionally along two-dimensional (2D) reduced graphene oxide (rGO) nanosheets and assembled into nanospheres on the nanosheets, resulting in a unique 2D/3D heterogeneous structure. This peculiar 2D/3D heterogeneous structure with intrinsic conductive network inside the rGO effectively enhances the EMW absorption performance of the heterostructure, with a minimum reflection loss (RL) value of −47.86 dB at 11.04 GHz at a thin thickness of 2.28 mm, and a minimum RL value of −38.64 dB at 15.28 GHz at a thinner thickness of 1.73 mm, and achieves a broadband effective absorption (RL < -10 dB) over 5 GHz, covering almost the whole Ku-band. The results reveal the influence of 2D/3D heterogeneous interfaces and the confinement effect of intrinsic conductive network on the improvement of EMW absorption performance, providing a new idea for the development of a new generation of ultrathin EMW absorption materials with high performance.