K2Ti6O13 whisker (PTW) reinforced AZ91D magnesium matrix composites were fabricated by a stirring casting method. The corrosion behavior of the composites was examined by potentiodynamic polarization and static corrosion tests. The surface morphology of the composites were investigated by SEM before and after corrosion tests. The results show that α-Mg grains are refined and the distribution of β phase become more uniform for the AZ91D magnesium matrix with the addition of PTW. Meanwhile, the morphologies of β phase in composites with different contents of PTW are obviously different, namely the skeletal structure of β phase is gradually transformed into partially detached honeycomb structure, and lamellar structure disappears. It can be deduced that fine α Mg grains and the decrease of β phase volume fraction lead to the inhibition of galvanic corrosion, thus increasing the corrosion resistance of the AZ91D magnesium composites.