In order to eliminate the pores and shrinkage porosity in the casting aluminum silicon alloy, and improve the microstructures and the mechanical properties of the alloy, large size Al-Si alloy pipes were prepared with gravity casting. A new wedge pressing process for compressing large scale porous tubular performances was adopted on the basis of the idea of accumulating local small deformation as to integral forming. The rule of the wedge pressing process and microstructures and the mechanical properties of the alloy were investigated with hardness tests, tensile tests and SEM. The results have shown that the wedge pressing process can markedly eliminate the pores and shrinkage porosity in gravity casting performances. The coarse primary silicon was crushed, and most of the eutectic silicon was refined and distributed uniformly in the matrix. The mechanical properties and density of the gravity casting increased from 112 MPa, 1.2% and 2.48g/cm3 to 203 MPa, 5.2% and 2.68g/cm3 respectively after the reduction of 40%.