A surface treatment of a magnesium alloy includes preparing a substrate of magnesium alloy, micro-arc oxidizing the substrate of magnesium alloy, forming an oxide layer with a hydroxyl group on the substrate of magnesium alloy, silylizing the oxide layer of the substrate of magnesium alloy with the oxide layer, by soaking the substrate of magnesium alloy in a processing solution with a silyl group-containing compound for 1-300 minutes, and placing the substrate of magnesium alloy with the silylized oxide layer at 70-200° C. for 1-300 minutes, allowing a condensation reaction to occur. The manufactured surface-treated magnesium alloy shows a decreased degradation rate in vivo.