There is considerable demand for high-performance, low-cost, and rare-earth-free magnesium alloys in several industrial applications because of their energy conservation potential. However, the mechanical properties of the currently available rare-earth-free magnesium alloys cannot satisfy the industrial requirements. Therefore, a novel rare-earth-free magnesium alloy with high strength, excellent ductility, and good formability must be urgently developed. In this study, the microstructure and mechanical properties of the Mg-2.5Sn-2Ca-xAl (x=2, 4, and 9, mass fraction, %) alloys in the as-cast and extruded states when different amounts of Al content are added are systematically studied. As indicated by the results, the strength and elongation of the alloy decrease and increase, respectively, with the increasing Al content. The yield strengths of the Mg-2.5Sn-2Ca-2Al, Mg-2.5Sn-2Ca-4Al, and Mg-2.5Sn-2Ca-9Al alloys are approximately 370, 325, and 290 MPa, respectively, and their elongations are approximately 6.2%, 11.0%, and 12.0%, respectively. The type and content of the nanosecond phase of the Mg-Sn-Ca-based alloy changed because of the addition of the fourth type of Al element. High-density G.P. zones and a second phase of Mg17Al12 can be observed in the extruded Mg-2.5Sn-2Ca-2Al and Mg-2.5Sn-2Ca-9Al alloys, respectively; however, nanophase precipitation cannot be observed in case of the extruded Mg-2.5Sn-2Ca-4Al alloy. The high-density G.P. zones hinder the growth of the recrystallized grains more efficiently than the Mg17Al12 nanophase; thus, the recrystallized grains of the extruded Mg-2.5Sn-2Ca-2Al alloys are finer (approximately 0.5 μm) than the extruded Mg-2.5Sn-2Ca-9Al alloy. Based on TEM images, high-density dislocations can be observed inside the extruded Mg-2.5Sn-2Ca-2Al alloy grains and G.P. zones can be observed toward the side of the dislocations; thus, the high density subgrain lamella structure is retained in the alloy (lamella thickness: 0.2~1.0 μm). The movement of the newly generated dislocations is inhibited by the large number of G.P. zones and residual dislocations, increasing the yield strength and decreasing the plasticity of the Mg-2.5Sn-2Ca-2Al alloy. The Mg17Al12 nanophase that was formed in the Mg-2.5Sn-2Ca-9Al alloy because of the addition of high Al content exhibits a weak ability to hinder the movement of the dislocations, resulting in low-density residual dislocation. Therefore, the Mg-2.5Sn-2Ca-9Al alloy, exhibits a large grain size, low yield strength and high plasticity.