Abstract: Based on the investigation of the rheological and thixotropic behavior of semi-solid AZ91D slurry, a thixotropic model describing the apparent viscosity of the slurry was established by the curve fitting method. A specific program used for calculating the apparent viscosity of AZ91D slurry during thixodiecasting was developed and embedded into the CastSoft simulating program. The flow field and the temperature field during thixo-diecasting process were simulated using the improved program. The results show that the filling speed and initial temperature of the slurry have great effects on the flow patterns of semi-solid AZ91D slurry during thix-filling, the optimum filling speed of slurry is between 2 and 5 m·s^-1 and the optimum temperature is from 570℃ to 580℃.