The as-cast Mg-9Li-3Al-xSi(x=0,0.1,0.5,1.0 wt%)alloys were prepared in a vacuum induction furnace with the protection of argon. The alloys were then investigated by OM,SEM,XRD and mechanical properties test. Results exhibit that the as-cast Mg-9Li-3Al-xSi alloys are consist of α-Mg, β-Li, Mg17Al12 and phase. The addition of Si leads to the formation of new phase (Mg2Si) and the refinement of the grain. But if the content of Si is too high, α-Mg phase will coarsened, there are block and Chinese character shaped Mg2Si phase existing in the phase boundary. The temsile strength of alloys initially enhance and then decrease while the elongation declines with the increase of Si content. When Si addition is 0.1wt%, the tensile strength of the alloy comes to the maximum of 182.5Mpa, enhanced by 59.6% compared to the Mg-9Li-3Al alloy, while the elongation is 12.1%.