Corrosion resistance of as-extruded and as-annealed binary Mg-xLi (x=1, 3, 5, wt%) alloys was investigated via immersion tests, electrochemical tests, corrosion morphology observation, and scanning Kelvin probe tests. Results reveal that the annealing twins form in the as-extruded alloys after annealing at 350 °C for 4 h, and the corrosion resistance of the as-annealed alloys is obviously worse than that of as-extruded alloys in 0.1 mol/L NaCl solution. Typical filiform corrosion appears in as-extruded alloys at the initial corrosion stage, according to the analysis results of scanning electron microscope and laser confocal 3D morphology observation. Some corrosion pits are gradually formed with prolonging the corrosion duration. The rapid corrosion damage occurs on the as-annealed alloy surface, and meanwhile the relatively large and deep corrosion pits appear on the alloy surface. Besides, the distribution of isopotential line of corrosion indicates the occurrence of preferential corrosive degradation in the annealing twin areas. Thus, the micro-galvanic effect is formed between the annealing twins and parental grains with annealing twins as anode and parental grains as cathode, which decreases the corrosion resistance of as-annealed Mg-Li alloys.