Magnesium metal batteries (MMBs) have attracted increasing attention due to the high volume specific capacity (3833 mAh/cm³) and high safety of Mg metal anode. Nevertheless, the high polarization effect induced by Mg²⁺ inhibits its diffusion in solid phase and therefore limits the specific capacity of MMBs. Li⁺/Mg²⁺ dual-salt electrolyte has been proposed to circumvent the sluggish diffusion of Mg²⁺ in solid phase, which enables Li⁺ to replace Mg²⁺ to drive the cathode reaction. In this work, the electrochemical performance of CoS₂ as conversion cathode of MMBs is studied based on different Li⁺/Mg²⁺ dual-salt electrolytes, and the effects of Li-salt concentration and discharge-charge voltage range on cycling stability are analyzed. The strategy of Li-salt additive remarkably promotes the conversion kinetics of CoS₂ based MMBs. By developing charge voltage to 2.75 V, the cycling stability of Mg-CoS₂ cell in LiCl-APC electrolyte is significantly enhanced. Its specific capacity can be maintained at 275 mAh/g after 150 cycles, which is much higher than that (33 mAh/g) under the protocol of 2.0 V charge voltage. It is found that the capacity degradation of MMBs is related to the irreversible reaction of CoS₂ and generation of Co₃S₄ at the charge potential of 2.0 V. And the dissolution of Co and S elements from active species aggravates the irreversible loss of capacity. This study provides an electrochemical activation solution to the development of transition metal sulfides in conversion type MMBs.