The invention relates to a polylactic-acid-modified 
magnesium alloy medical 
composite material and a preparation method thereof. The method comprises: 1) performing 
micro arc oxidation treatment on a surface of a 
magnesium alloy substrate, and performing 
coupling treatment to obtain a silicane 
coupling substrate; 2) adding a surfactant to a 
chitosan solution, and performing uniform mixing to obtain a 
chitosan mixed liquid, and dissolving 
polylactic acid in trichloromethane to prepare a 
polylactic acid solution, and adding the 
polylactic acid solution in the 
chitosan mixed liquid while performing stirring, and performing high speed emulsification to obtain a film-
casting liquid; and 3) 
coating the surface of the silicane 
coupling substrate with the film-
casting liquid, removing the 
solvent and the surfactant to obtain a dry film layer, and performing 
thermal insulation at 80-90 DEG C to obtain the 
composite material. The 
composite material is excellent in biological compatibility, surface activity and mechanical properties. The degradation rate of the material can be controlled by adjusting the thickness of a polylactic acid and chitosan film layer. After the material is implanted to a 
human body, the degradation product can be absorbed by the 
human body or excreted outside the body with 
metabolism, so that secondary operation is not required.