Magnesium alloys are considered as “difficult to plate” metal. The pretreatment processes play a key role for plating. The pretreatments of alkaline cleaning, chromic acid etching and hydrofluoric acid activating were carried out and the microstructures investigated by SEM and EDAX in detail. The hydrofluoric acid activating time of 8 minutes was determined by Potential-Time curve. The experimental results showed that the alkaline cleaning moved the grease and oils off the substrate surface, but there was no apparent surface morphology change. The chromic acid etching was a very critical step, and the optimum etching time of 1 minute was chosen; the eutectic α was severely etched, and the chromic compounds were mainly formed on the surface. During the hydrofluoric acid activating, the white activating product of magnesium fluoride was preferentially generated on the β–phase, and obvious morphology change has been observed on the alloy substrate surface.