The FDM numerical simulation software View Cast system was employed to the counter-pressure casting of aluminum alloy large-scale thin-section casting. By analyzing the mold filling and solidification, the distribution of liquid fraction, temperature field were studied. The potential shrinkage defects were predicted to be formed at the top of the casting. A solution towards reducing such defects has been presented. The feeding capacity of the riser was improved. Analysis on the shrinkage proved that the improved riser is an effective method for reduction of defects.