Mg58Al42 hydrogen storage alloy was prepared by mechanical alloying (MA) method and its microstructural characteristics and electrochemical properties such as discharge capacities and corrosion mechanisms were investigated by XRD, PARM273A and M5210 electrochemical apparatus. The results show that the X-ray diffraction peaks have a deviation and broadening with increment of ball milling time because of fine grain size and internal stress. The discharge capacities of the present alloy increase at the beginning of ball milling and afterwards decrease with increment of ball milling, discharge capacity of which is biggest when it is milled for 20 h. There are passivation phenomena in potentiodynamic polarization curves. The corrosion current densities of the present alloy increase with time at the beginning of ball milling and afterwards decrease. The electrochemical impedance spectroscopies(EIS) of the present alloy are composed of single capacitive loop and electrode reaction is controlled by charge transfer at alloy/electrolyte interface. The CPE-T of alloy increases. It showed that the specific surface areas of the alloy increase with ball milling time