The work aims to explore best technological parameters of preparing aluminum-magnesium alloy by electrodeposition with AlCl3 and LiAlH4 as solutes, and benzene and tetrahydrofuran as solvents. Mg ions (Mg2+) of certain concentration were obtained by pre-electrodeposition using a pure Mg anode in AlCl3-LiAlH4-benzene-tetrahydrofuran (THF) system, so as to prepare aluminum-magnesium alloy coating by electrodeposition. Effects of technological parameters on morphology, grain size and magnesium content were investigated by changing parameters including current density, electrodeposition time, pre-electrodeposition time and times. Coating morphology, composition and content were tested and analyzed by using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Principal components of the coating were Al3Mg2 and Al12Mg17. The surface state was the best and grain size was about 1~2 mm provided with current density of 16.67 mA/cm2. The more the pre-electrodeposition times were, the more Mg2+ were obtained by pre-electrodeposition, and the higher the magnesium content (of which mass fraction might be up to 2.93%) was. Magnesium content of the coatings increases as the pre-deposition time increases in a certain range. The higher the pre-electrodeposition frequency is, the more dissolved Mg2+ ions are, the higher magnesium content of aluminum-magnesium alloy coatings is. Improvement of magnesium content of the coatings will be inhibited if electrodeposition time is too long. The magnesium content is in direct proportion to electrochemical impedance, corrosion potential and corrosion resistance of the coatings.