The microstructure, mechanical properties and corrosive behavior of the 3.5wt%NaCl solution of the as-cast and the as-extruded Mg-4.5Zn-4.5Sn-2Al alloy were investigated. The results show that the average grain size of the as-cast alloy is 183 μm and that of the extruded alloy is 9 μm. Compared with as-cast alloy, the tensile strength of the extruded alloy increases from 209 MPa to 354 MPa, the yield strength increases from 157 MPa to 216 MPa, and the elongation reaches about 19.6%. The tensile fracture of the extruded alloys exhibits ductile fracture. Static immersion corrosion tests and electrochemical measurements show that the corrosion resistance of the extruded alloy is obviously inferior to that of as-cast alloy.