Instead of the chromate salt chemical conversion treatment for aerospace magnesium alloys, a technology of the rare earth salt chemical conversion is studied and the corrosion resistance is evaluated as well. These micrographs of specimen surface are investigated by metallographic microscope. The corrosion behaviors of magnesium alloy and conversion coating are assessed by means of potentio- dynamic polarization curves and immersion tests. The experimental results indicate that a yellow-brown conversion coating is formed subsequently on the specimen surface, the conversion technology condition is a mixture solution including 3 g/L CeCl3·H2O、5g/L KMnO4 and the other at room temperature. The formation of the rare earth conversion coating and the composition of the coating are discussed by the principle of thermodynamics. The research results indicate that the composition of the coating includes MgO,CeO2,MnO2,Mg (OH)2 and Ce (OH)4. During the initial of the rare earth salt conversion, mass gain is a linear growth with time. After 30min, mass gain is approximately unchanged.