Abstract: The influence of hot extrusion on the microstructure, mechanical properties and corrosion resistance of as-casted Mg-3.5Al-3.5Ca-0.6Mn alloy was investigated. The results show that hot extrusion could refine the structure of as-casted alloy with grain size from 80 μm to 6 μm after hot extrusion. The continuous network-like large second phases on grain boundaries were severely broken into micron even nanoscale particles and distributed like ribbons in the matrix. The ultimate tensile strength and elongation of hot extruded alloy were 313.9 MPa and 9.3%, which was higher than that of as cast alloy by 153.8% and 564.3%, respectively. After hot extrusion, the self-corrosion potential and arc radius of high-frequency capacity increased, while the self-corrosion current density and average hydrogen evolution rates decreased. The good corrosion resistance of matrix and grain boundaries caused by grain refinement, corrosion barrier caused by ribbon-like secondary phases and relatively stable and dense corrosion product film after grain refinement were responsible for higher corrosion resistance of hot extruded alloy.