This paper focuses on the corrosion behavior of Mg-5.7Gd-1.9Ag alloy sheets with high strength-ductility synergy in the 3.5 wt% NaCl solution prepared by a combined route of rolling and aging. Electrochemical measurements indicate that the rolled sheet exhibits a significantly improved corrosion resistance as compared to its as-cast counterpart. Subsequent aging, as a widely used strengthening method, decreases corrosion resistance to a certain extent. However, the aged sheet still has enhanced corrosion resistance as compared to its as-cast counterpart. The observed corrosion migration in the present Mg-5.7Gd-1.9Ag Mg alloy mainly stems from the evolution of second phases, but not the grain size and texture. A small number of the micron β phase particles is incapable of promoting severe galvanic corrosion and thus endows the rolled sheet with good corrosion resistance, while the massive submicron β phase particles along grain boundaries and dense γ′′ nanoplates within grains in the aged sheet bring out severe galvanic corrosion and thus accelerate corrosion.