The apparent viscosity model of the semi-solid A356 aluminum alloy, based on fitting of experimental data obtained by the Couette type viscometer, was developed in the paper. The commercial package CastSoft6.0 coupled with the model was used to simulate the mould filling of the semi-solid A356 aluminum alloy in the key-shaped component with iron cores. The simulation results showed that the position of the iron cores has an important effect on the filling of the semisolid slurry, and it is easy to obtain the completely filled key-shaped component when the iron cores were near to the inlet. The filling tests verified that the simulation results have good agreement with the experimental results. The fitting results indicated that the developed apparent viscosity model is practical and feasible and it can be used to simulate the mould filling process of the semisolid A356 aluminum alloy slurry. Also the parameters were optimized and the optimum parameters are as follows: the inject pressure is more than 15MPa, the inlet velocity is more than 1.73m/s and the forming temperature is over 585 °C.