Electrochemical behavior of AZ31 wrought magnesium alloy activated by hydrofluoric acid (HF) of different concentration was studied in an electroless nickel plating bath, using mixed potential-time curves, polarization curves and electrochemical impedance spectrum. Changes of morphologies of AZ31 samples before and after the HF activation were observed by scanning electron microscopy (SEM). The results show that the time in which the activated samples were coated completely by Ni-P layers extends with the increase of the concentration of HF. The onset reduction potential of Ni2+ in the plating bath shifts toward positive direction with an increase in the concentration of HF. The charge transfer resistance of electrode/bath interface reaction increases with an increase in the concentration of HF, and the interface reaction between the electrode and the bath is the rate-controlling step during the initial deposition stage. The shedding of Mn-Al particles in the AZ31 magnesium alloy matrix during the HF activation process results in the defect formation of the fluoride film, and the higher concentration of HF, the less defect formed on the fluoride film.