Magnesium alloys are more and more researched and used as engineering materials in recent years. In the paper, the AZ91D casting magnesium alloy was as basic material by the proper chemical pre-treatment and then was coated with epoxy resin (EP) coatings modified with polyurethane (PU) for protection. The coatings’ adhesion strength, hardness, and corrosion resistance were studied by electronic universal tester, pencil scratch hardness tester, immersion test and electrochemical test. The effect of polyurethane addition on the properties of the coating was ales investigated. The results show that PU / EP ratio of 20% mass fraction can make the basic alloy good mechanical properties and excellent corrosion resistance. The interpenetrating network formed between the epoxy resin modified with polyurethane is of more excellent mechanical and corrosion resistance than pure epoxy resin to protect the magnesium alloy matrix.