In the present work,a double-pass continuous expansion extrusion forming (CEEF) process was proposed for an Al-Mg-Si alloy,in which the diameter of rods was gradually expanded.The microstruc-tural evolution,mechanical properties and deformation characteristics were investigated by utilizing microstructural observations,mechanical testing and a finite element method coupled with a cellular automata model.The results showed that the strength and ductility of the double-pass CEEF processed Al-Mg-Si alloys were improved synchronously,especially in artificially aged alloys.The grain size of the processed Al-Mg-Si alloy rods was refined remarkably by continuous dynamic recrystallization (CDRX)and geometric dynamic recrystallization (GDRX),and the homogeneity of microstructure was gradu-ally improved with increasing number of processing passes.The artificially aged alloy processed with double-pass CEEF and water quenching contained fine (sub)grains and high-density dislocations,which resulted in more needle-shaped β\" precipitates and a larger precipitate aspect ratio than the as-received and air-cooled CEEF alloys owing to the different precipitation kinetics.The severe cumulate strain and microshear bands were found to accelerate CDRX and GDRX for grain refinement between adjacent posi-tions of the parabolic metal flow due to the special temperature characteristics and large shear straining during the CEEF process.