Abstract: The effect of forming temperature, injection pressure and piston velocity on the filling ability of semi-solid A356 aluminum alloy slurry were investigated. It is shown that a higher forming temperature helps to the good filling ability, and injection pressure and piston velocity have important effect on the filling ability. The higher the injection pressure, or the higher the piston velocity, the better the filling ability is. Besides the thickness of a sample piece has great effect on the filling ability, and the thicker the sample piece, the better the filling ability. The results also show that the mierostructure distribution of A356 semisolid alloy is homogeneous, which is helpful to high quality castings.