Abstract: By field exposure test, atmospheric corrosion tests of AZ31 magnesium alloy were conducted in Xisha Islands for 4 a. The surface and cross-section morphologies of corrosion products as well as the corrosion morphologies of the alloy after removing corrosion products were observed by scanning electron microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction analysis were used to obtain the element content and phase composition of corrosion products. The results indicate that the alloy undergoes severe corrosion. The average corrosion rate is 11.95μm·a^-1. Cl^- and CO₂ play important roles in the corrosion process. The Cl^-—containing absorbed electrolyte layers will destroy the oxidation film and induce anodic dissolution of the alloy. While CO₂ dissolved in the absorbed electrolyte layers tends to neutralize the alkali formed in the cathodic area and reacts with Mg (OH) 2 to form Mg₅(CO₃)₄(OH)₂·xH₂O. The surface corrosion products restrict the transport of CO₂ and Cl^- to the surface of the alloy, so the inner layer of corrosion products is mainly composed of Mg(OH) 2.