Electrochemical corrosion of AZ31 magnesium alloy in the NH₄⁺-SO₄²⁻-Cl⁻ environment is studied. Effect of NH₄⁺ overshadows that of Cl⁻ as the (NH₄)₂SO₄ concentration is 0.005 M or higher, yielding an evolution from localized corrosion to uniform corrosion. Acceleration effect of NH₄⁺ can be attributed to that (ⅰ) NH₄⁺ dissolves the inner MgO and hinders the precipitation of Mg(OH)₂ and (ⅱ) the buffering ability of NH₄⁺ provides H⁺, enhances the hydrogen evolution, and expedites the corrosion process. The latter is demonstrated as the dominant factor with the results in unbuffered and buffered environments. The severe corrosion and hydrogen process in NH₄⁺-containing solution results in a high H_ads coverage and yields an inductive loop within the low frequency. Meanwhile, SO₄²⁻ is helpful in generating cracked but partially protective corrosion products, while Cl⁻ could broaden the corrosion area beneath the corrosion product.