Abstract: Based on the behavior of filling pressure during semisolid alloy slurry filling process, a mathematic model of slurry flow coupled with apparent viscosity was established. The filling process and filling pressure of thixotropic Al alloys and steel were simulated. It is demonstrated that the filling pressure gradually increases with the course of the filling, in response to flow resistance and gravity and with a range of 0.12 MPa. Semisolid alloy slurry flows in a stable and laminar-type manner within the range of filling velocity. Compared with thixotropic Al alloys, the high melting point of steel leads to the decrease of filling temperature easily, so the increase of apparent viscosity results in casting defects. The simulation results of filling pressure and the experimental results of semisolid brake pump casting show that semisolid casting filled along the radial presents good forming quality.