The use of high damping Al alloy is of great significance for lightweight components or equipment and shock absorption and noise reduction. However, due to the low intrinsic of Al alloy, it is one of the development directions to improve the damping properties of Al alloy by alloying and introducing high damping second phase. The influence of rare earth La on the microstructure, damping and mechanical properties of cast aluminum alloy containing 4 wt.%Sn is studied. The damping improvement mechanism is discussed from the aspects of La on the morphology and distribution of Sn, and the influence of La on the wetting characteristics of Sn and aluminum alloy matrix. The results show that when the La content is 0.2 wt.%, the grain size of 4 wt.%Sn AlMgMnSi alloy is improved, and the β-Sn from large granular to fine, diffuse distribution.The La content increase to 0.83 wt.%, the grain size becomes larger and the bulk rare earth compounds appear at grain boundary. The addition of La can effectively improve the damping performance of the cast aluminum alloy containing 4 wt.%Sn. When the La addition amount is 0.2 wt.%, the damping property of AlMgMnSi with 4wt.%Sn and ZL102 alloy increase by about 70% and 100%. Respectively, La improves the wettability of β-Sn and aluminum matrix, make the β-Sn diffuse and fine distribution, thus improving the phase interface damping, which is the reason for the high damping performance of the samples.