It is of great engineering significance to improve the damping and mechanical properties of cast aluminum alloy at the same time. In this paper, the mechanical properties and damping properties of AlSi10Sn4MgX(x=0~4.0 wt%) alloy have been studied, and the improving mechanism of the damping and mechanical properties have been analyzed by researching the effect of Mg on the wettability of Al/Sn and alloy structure. The results of mechanical property test show that the tensile strength of the AlSi10Sn4MgX alloy increase with the increasing of Mg content in the range of 0-1.0 wt%, and the fracture elongation change slightly at the same time. When the Mg content increase to 1.0wt%, the tensile strength of the alloy is 161 MPa, and the fracture elongation is 3.4 %. Continue increasing Mg content, the strength of the samples increases observably, but the fracture elongation decreases significantly. The damping properties of the alloy firstly increase and then decrease with the increasing of Mg content. The alloy containing 1.0 wt% Mg have the better damping properties, and also have an excellent damping property at high temperature and high frequency. In this content the damping loss factor tanh at 25 ℃/1Hz, 200 ℃/1 Hz and 25 ℃/40 Hz is 0.038, 0.058 and 0.069, respectively. Further research shows that the addition of a small amount of Mg can effectively improve the wettability of Al/Sn and thus the small-sized β-Sn phase distributes along grain boundaries dispersedly. Meanwhile, a small amount of Mg can also modify and spheroidize the eutectic Si phase in AlSi10Sn4MgX alloys and form Mg2Si strengthened phase. Both the above two aspects are the main reasons for the high damping and mechanical properties of the alloy.