Mg-rich primer as a new type protective coating provides cathodic protection for Al alloy. In this paper, akind of phosphatizing surface treatment on Mg particles was studied in order to improve the protectiveperformance of Mg-rich primer. After treated with phosphoric acid, a protective magnesium phosphatelayer was formed on the surface of Mg particles, which had no negative influence on the cathodic protec-tion of the Mg-rich primer for Al alloy. The coating resistance of the treated Mg-rich primer was biggerthan that of untreated primer, meanwhile the coating capacitance of the treated Mg-rich primer was**aller than that of untreated primer, suggesting that the barrier effect of the primer was improved andthe lifetime was extended. The magnesium phosphate layer could reduce the consumption rate of Mgparticles. Meanwhile, the phosphate radicals transported to Al alloy substrate to form a product layercomposed of magnesium phosphate and aluminum phosphate on the substrate surface, which decreasedthe corrosion rate of Al alloy and improved the protective performance of the primer.