The formation and growth of surface film on AZ63 (Mg–6Al–3Zn) magnesium alloys were studied in 2 M MgSO4 aqueous solution using electrochemical methods. Surface examinations were carried out using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and microscopic Fourier transform infrared spectroscopy. Experimental results show that the corrosion current decreases with prolonged immersion time, the three stages of hydrogen evolution rate correspond with the growth processes of the surface film on AZ63 magnesium alloys, and charge transfer resistance increases with the accumulation of corrosion products. A layer of MgO with sulfate salt grains underneath seems smooth at first. However, a few surface micro-cracks caused by inner stress appear on the smooth base film after 5 h of immersion, followed by the aggregation of spherical grains and the formation of cracks after 12 h. It is suggested that sulfate salt, carbonate salt, and hydroxide of magnesium should be the main composition of the surface film.