Magnesium (Mg)-ion batteries with low cost and good safety characteristics has attracted a great deal of attention recently. However, the high polarity and the slow diffusion of Mg 2+ in the cathode material limit the development of practical Mg cathode materials. In this paper, an anion-rich electrode material, NiS 2 , and its composite with Ni-based carbon nanotubes (NiS 2 /NCNTs) are explored as the cathode materials for Mg-ion batteries. These NiS 2 /NCNTs with excellent Mg 2+ storage property is synthesized by a simple in situ growth of NiS 2 nanoparticles on NCNTs. NiS 2 with both a large regular cavity structure and abundant sulfur-sulfur (SS) bonds with high electronegativity can provide a large number of active sites and unobstructed transport paths for the insertion-disinsertion of Mg 2+ . With the aid of 3D NCNTs skeleton as the transport channel of the electron, the NiS 2 /NCNTs exhibit a high capacity of 244.5 mAh g -1 at 50 mA g -1 and an outstanding rate performance (94.7 mAh g -1 at 1000 mA g -1 ). It achieves capacitance retention of 58% after 2000 cycles at 200 mA g -1 . Through theoretical density functional theory (DFT) calculations and a series of systematic ex situ characterizations, the magnesiation/demagnesiation mechanisms of NiS 2 and NiS 2 /NCNTs and are elucidated for fundamental understanding.