► ECAPed UFG AZ91D Mg alloys shows significant weakened corrosion resistance. ► The corrosion behaviours of the ECAPed samples changed into severe uniform corrosion gradually. ► Crystalline defects weakened the corrosion resistance of the α-phase matrix is the first factor. ► Refined β-phase particles losing corrosion barrier in α-phase matrix is the second factor. Effect of microstructure change on corrosion behaviour of equal-channel angular pressed (ECAPed) AZ91D Mg alloy was investigated. The ECAPed alloy with ultra-fine grained (UFG) α-phase matrix and refined β-phase particles displays a significantly lower corrosion resistance, resulting in more pits after in-situ corrosion, higher mass-loss rate immersed in NaCl solution, larger I corr values in polarization curves and lower fitted R t values in EIS plots. Two factors weaken the corrosion resistance: the first is the strains-induced crystalline defects providing the α-phase matrix more corrosion activation, the second is the refined β-phase particles losing barrier to the corrosion propagation in α-phase matrix.