Carbon nanotubes (CNTs)-enforced AZ31 magnesium-based composites were fabricated by stirring-casting under the protection of Ar. Their microstructure was observed and the effect of CNTs addition on the corrosion resistance of AZ31 magnesium alloy was studied by immersing all the samples in 3.5wt% NaCl solution. The change of pH value was tested during the immersing progress. The fractographs were observed and analyzed via scanning electron microscope. Corrosion resistance mechanism of CNTs/magnesium composites was researched. The results show that CNTs addition can increase the corrosion resistance of the composites effectively. Corrosion rate changes from 3.2068 mg/(m2·s) to 1.1069 mg/(m2·s) with 1.5wt% CNTs addition