Mg-9Li-3Al-xSr alloys were prepared in vacuum furnace under the protection of argon atmosphere. Effects of Sr content on microstructure and mechanical properties of as-cast Mg-9Li-3Al alloys were investigated. The results indicate that α-Mg and β-Li phases exist in all alloys. The addition of Sr causes grain refinement of the test alloys and new Al4Sr phase is formed in the alloys. The mechanical properties of alloys initially increase and then decrease with the increase of Sr content. Sr has obvious refining effect on the alloys and the distribution of Al4Sr phase is much more uniformly when Sr content is 2.5wt%,. The as-cast Mg-9Li-3Al-2.5Sr alloy exhibits an optimum combination of mechanical properties with the ultimate strength, yield strength and elongation of 186MPa, 149MPa and 11% respectively, improving obviously compared with Mg-9Li-3Al alloy. With the continuing increase of Sr content, the mechanical properties of the test alloys decrease.