Carbon fibre reinforced magnesium-based composites were fabricated by gas press infiltration method. Three kinds of magnesium alloys matrixes were employed. The fractographes of the composites were characterized by SEM, and the interfacial areas of C/Mg were characterized by TEM. The results showed that the reactions in the interfacial areas were affected by the content of Aluminum in the matrix. As the increasing content of Aluminum, the reactions became more intensive. Also, the interfacial bonding strength and microstructure were seriously influenced by the reactions. A weak-bonding interface was found in the C/c.p.Mg composite, however, the strong-bonding interfaces were found in both Cf/AZ31 and Cf/AZ91 composites.