 Magnesium (Mg) is a promising anode material for primary Mg batteries because of its outstanding characteristics, such as abundance, light weight, and low cost. However, self-corrosion and delayed action of Mg alloy in aqueous solution limit its performance and reduce the energy density. In this work, the influence of NaF–Na3PO4 on electrochemical behaviors of Mg alloy in composite solution is studied by electrochemical impedance spectroscopy, galvanostatic discharge, and linear sweep voltammetry. The morphology and microstructure are analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy. Impedance studies show that NaF and Na3PO4 are good inhibitors with the inhibition efficiency reaches 98.8%. Moreover, the delayed time is reduced to 0.08 s, despite that soaking time length is extended to 16 days. A Mg–MnO2 cell demonstrates an excellent discharge capacity of 1539 mAh g−1 at the discharge current density of 5 mA cm−2.