The present invention provides a surface treatment method for magnesium alloy, which comprising the following steps: 1) preparation; 2) fusion and uniformly coating; 3) heat diffusion, and 4) finish; so a coating alloy is placed on a magnesium alloy substrate, and the magnesium alloy substrate is heated so that the coating alloy is uniformly melted on the magnesium alloy substrate; when heating up to a preset temperature, the coating alloy generates heat diffusion on the magnesium alloy substrate; the coating alloy finally forms a corrosion-resistant hard layer on the magnesium alloy substrate. So, this invention features simple treatment process, stable structure and environmental-friendliness in a wide range of applications.