Stress-induced melt activation (SIMA) has a certain advantage during the preparation of semi-solid billet of Mg alloy. However, Equal-Channel Angular Pressing (ECAP) only can produce small billet by several channels to achieve large equivalent strain. To conquer this problem, the forward extrusion/(ECAP) method, a new process for preparing semi-solid Mg alloy was proposed. And the feasibility of forward extrusion/ECAP was analyzed. This new method simplified the processing procedure for providing commercial semisolid billets of Mg alloy. The experimental dies and experimental procedure were designed. The effect of extrusion ratio and equivalent strain on the microstructure of semisolid Mg alloy was investigated. It was found that when the extrusion ratio was larger than 10, the semisolid microstructure is refined, with increasing extrusion ratio, the microstructure is refined further.