The constitutive equation of semi-solid 6061 alloy was established according to the thermal simulation compression tests of semi-solid billet of 6061 alloy prepared by near-liquids semi-continuous casting method. Parameters of Bingham model, such as apparent viscosity, had been investigated based on the constitutive equation of semi-solid 6061 alloy. The software ANYCASTING was applied to simulate and analyze the semi-solid die-casting processes of semi-solid 6061 alloy. A well-designed runner and gating system of automobile aluminum alloy part were presented and the casting defect were investigated here utilizing these parameters by simulation. The results show that a well-designed runner and gating system is very important to secure good quality die castings through providing a homogenous mould filling pattern. Most defects of semi-solid casting products can be founded in the final solidification of the parts of casting.