The plastic forming of magnesium alloy is difficult, but the semi-solid material forming is a good method solved this problem. The mechanical model of the semi-solid materials was treated as that of the continuous porous materials in the high solid volume fraction. The upper bound theory applied for semi-solid metal plastic forming process was developed. The velocity discontinuities exist not only in the tangential component but also in normal component for the kinematically admissible displacement increment filed. The latter one was responsible for a change in solid volume fraction when the material passes the discontinuity. An upper bound analytical model and theoretical method of plastic forming process for semi-solid material has been proposed. The calculating formulas of deformed power were derived. It is theoretical basement to apply further for the practice technology analysis such as the plastic forming of magnesium alloy.