Mechanical properties of semi-solid casting are dependent on multiple processing parameters, and improper processing parameters will not only reduce mean data but also increase variations. The present study investigated the impact of parameters in slurry preparation and heat treatment on the yield strength and ductility of T6 heat-treated A356 Al-Si alloy using rapid slurry forming (RSF) semi-solid casting. The focus was primarily on the robustness of mechanical properties based on Taguchi design method. By analyzing signal-to-noise ratio and minimum value calculated from -3S, the optimum slurry preparation parameters and heat treatment parameters were determined to be no quench, enthalpy exchange material (EEM) temperature of 140 °C, EEM-to-melt ratio of 6mass%, stirring time of 18 s, solution heat treated at 520 °C for 2 h, and ageing heat treated at 190 °C for 6 h. In a small batch validation, the -3S yield strength and -3S elongation reach 256.1 MPa and 5.03% respectively, showing a satisfactory robustness. The hardness and microstructure of heat-treated samples with the best and worst properties were characterized to gain insight into the underlying mechanisms affecting the mean value and variations of mechanical properties.