The relationship between reductive decomposition of anion and the efficiency of magnesium deposition was investigated in magnesium bis (trifluoromethane sulfonyl) imide (Mg(TFSI)2) salt and triglyme (G3) solvent without and with magnesium ethoxy chloride (EtOMgCl) salt. Cyclic voltammetry, energy dispersive X-ray spectroscopy analysis, in situ infrared spectroscopy during deposition-dissolution of magnesium and Raman spectroscopy were conducted. Coulombic efficiency for magnesium deposition was also calculated by chrono amperometry. In Mg(TFSI)2/G3 (molar ratio of 1/15), magnesium was hardly deposited. In EtOMgCl/Mg(TFSI)2/G3 (molar ratio of 2/1/15), magnesium was deposited efficiently. The reduction of TFSIanion occurred in Mg(TFSI)2/G3 during magnesium deposition, while the reduction was suppressed in EtOMgCl/Mg(TFSI)2/G3. The addition of EtOMgCl salt can suppress the reduction of TFSIanion, leading to highly efficient magnesium deposition.