As a new generation of energy storage devices, magnesium ion batteries have higher theoretical volumetric capacity, more abundant storage capacity and environmentally friendly than lithium ion batteries. However, the excessive polarization of Mg2+ in the batteries makes it difficult for ions to intercalate/extract from the cathode, resulting in a poor matching of the developed cathode material with the electrolyte. Copper sulfide (CuS) is an ideal cathode material for magnesium ion batteries, which has an excellent theoretical specific capacity and environmental friendliness and wide availability. In this paper, it was synthesized as cathode materials for magnesium ion batteries through adding the surfactant Cetyltrimethyl ammonium bromide (CTAB) and adjusting the percent of ethylene glycol to 50% in hydrothermal synthesis process, which was named as CuS-C50. Owning to the complete nanoflower structure, the CuS-C50 exhibited a high specific capacity of 331.19 mAh/g at 50 mA/g current density in APC/THF electrolyte, and still kept a specific capacity of 136.92 mAh/g over 50 cycles at 200 mA/g. The results of morphology characterizations indicated that the complete nanoflower structure could provide more active sites and reduce the barriers of Mg2+ moving, eventually improving the charge/discharge performance of the CuS cathode materials for magnesium ion batteries.