Aqueous rechargeable magnesium-ion batteries with low cost of magnesium resources have a potential to meet growing requirements for electric energy storage resulted from the similar electrochemical properties to lithium. The Mg1.1Mn6O12·4.5H2O named as magnesium octahedral molecular sieves (Mg-OMS-1) owns nanobelt structures as a cathode material for aqueous magnesium-ion battery. The morphology and structure of Mg-OMS-1 are measured by X-ray diffraction, scanning and transmission electron microscopy. The mechanism of magnesium-ion insertion/deinsertion from this host material and the theory specific capacity of Mg-OMS-1 are determined by cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. Mg-OMS-1 displays an excellent battery behavior for Mg2+ insertion and deinsertion in the magnesium-salt aqueous electrolyte. The initial discharge capacity of Mg-OMS-1 electrode can reach 248.8 ± 0.5 mAh g−1 at 10 mA g−1 in the 0.2 mol dm−3 Mg(NO3)2 aqueous electrolyte. Even back to 10 mA g−1 after the rate performance, the discharge capacity can achieve 214.1 ± 0.5 mAh g−1. The specific capacity retention rate is 90.4% after cycling 200 times at 100 mA g−1 in the 0.5 mol dm−3 Mg(NO3)2 electrolyte with a columbic efficiency of 99.7 ± 0.1% in the 5 times experiments.