Mg-0.8Bi-0.35Al-0.35Ge alloys with different grain structures were prepared by rolling and extrusion processes, including homogeneous E-T2 alloy and heterogeneous R-T2 alloy. The effects of grain structure on the mechanical properties and corrosion behavior of deformed alloys were discussed. Results show that the E-T2 alloy exhibits a homogeneous equiaxed grain structure, whereas the R-T2 alloy exhibits a necklace bimodal grain structure. E-T2 alloy exhibits excellent mechanical properties with synergistic corrosion resistance, whose yield strength, ultimate tensile strength, elongation, and corrosion rate are 208.0 MPa, 274.0 MPa, 25.4%, and 0.12 mm/a, respectively. In contrast, although R-T2 alloy has higher yield strength (285.0 MPa), its corrosion resistance is inferior (0.35 mm/a). This is mainly related to the severe intergranular corrosion caused by the presence of cathode Mg₃Bi₂ phase and high-density dislocations at the grain boundary.