Due to the complicated geometry and thin wall of the stainless steel impeller, during the investment casting process, some casting defects such as shrinkage porosity and filling shortage are usually produced, which have a close relationship with the filling and solidification process. So, the software ProCast was used to study the filling and solidification process of impeller and the numerical simulation results were compared with the experimental results in this paper. The following results can be acquired: 1. when the pouring temperature is 1550℃ and the casting speed is 0.75 m/s, the investment casting impeller can be filled to avoid the filling shortage defect; 2. although the appropriate pouring temperature and casting speed can avoid the filling shortage defect, but it can not avoid the shrinkage porosity defects in the impeller casting; 3. to pre-place certain cold iron in the middle of the impeller casting mold according to its structural characteristics can eliminate the shrinkage porosity defects. When the height of the pre-placed cold iron is 1/3 of the height of the inner cavity of impeller mold, the effectiveness in elimination the defects in the castings is the best thereby a high quality impeller can be acquired.