Abstract: The microstructure of A356 wheel casted in low pressure was analyzed by scanning electron microscopy, metallographic microscope and tensile test. The characterization method of Si segregation was established and the influence of SDAS and the average area of Si particles on the mechanical properties of the casting was analyzed. The results indicate that the degree of Si segregation can be well characterized by eutectic area fraction, the average area of Si particles and aspect ratio; eutectic area fraction, the average area of Si particles, aspect ratio and SDAS at R angle in the middle of hub spoke are 34.83%. 11.33μm², 2.46 and 56.89 μm, respectively, and the corresponding UTS, YS and EL values are 229.02 MPa, 152.67 MPa and 5.34%, respectively; the higher the degree of Si segregation of the hub is, the worse its mechanical properties are. SDAS and Si segregation should be both taken into consideration in the analysis of the tensile properties of the casting. This paper gives some specific measures to solve the problem of Si segregation.