Amorphous electrode active material, nickel hydroxide powder codoped with PO43- and Mg2+ was synthesized by the method of quick cryoprecipatation. Its microstructure and electrochemical performance were studied. Results show that the shape of the amorphous sample codoped with 5wt% PO43- and 2wt% Mg2+ is abnormal, the randomness is strong and it contains a large amount of crystal water up to 31%. As the positive active material of MH-Ni battery, the discharge specific capacity of the codoped sample is 347 mAh·g-1, and the discharge mean voltage reaches to 1.29 V. The discharge rate has a little effect on the discharge specific capacity of the electrode sample. The stability of electrode material is good after the 50 times circulation of charging and discharging and the capacity decrement is only 3.5%; the proton diffusion coefficient is 9.22×10-10 cm2·s-1 and the electrochemical impedance in the electrochemical reaction is small. The electrochemical performances of the sample are much better than those of β-Ni(OH)2 applied in MH-Ni production.