Using stir casting method, fly ash cenosphere/AZ91D composites were fabricated by adding fly ash cenospheres into magnesium alloy melt, and the microstructure uniformity, phase composition, mechanical properties and damping properties of the composites were studied. The results showed that the fly ash cenospheres distributed uniformly in the matrix, without segregation phenomenon. In the preparation process of the composites, the cenospheres reacted with the magnesium alloy melt and were filled in by the melt. Mg₂Si and MgO phases formed in the reaction. Compared with the matrix alloy, the mechanical properties and damping properties of the composites are obviously improved. The cleavage fracture occurs when the composites fail, indicating that the fracture is brittle fracture. The walls of the cenospheres are torn in the process of fracture. The damping mechanisms of the composites are mainly dislocation damping and interface damping.