High concentration magnesium borohydride/tetraglyme electrolyte for rechargeable magnesium batteries is simply prepared by dissolving inorganic magnesium salt Mg(BH4) in tetraglyme (TG) ether solvent with good safety. 90 °C heating treatment is performed in the preparation process and LiBH4 as a chelating agent is added to improve the electrochemical performance. Mg deposition–dissolution performance and the electrochemical window of the electrolyte on non-inert stainless steel (SS), nickel (Ni), copper (Cu) electrodes and inert platinum (Pt) electrode are systematically studied by cyclic voltammetry and constant current discharge–charge measurements. 0.5 mol L−1 heated Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L−1) solution shows good electrochemical performance with 2.4 V (vs. Mg RE) anodic stability on stainless steel, close to 100% Mg deposition/dissolution efficiency and high cycling reversibility. Furthermore, the reversible electrochemical process of Mg intercalation into Mo6S8 cathode with excellent cycling performance in the electrolyte indicates the feasible application in rechargeable magnesium batteries.