Abstract: Many experimental phenomena show that the primary Si content affects the primary crystal silicon distribution in hypereutectic Al-Si alloy functionally gradient matrial (FGM) prepared by centrifugal casting. To control the distribution of the primary crystal silicon, the influence is studied. The critical primary Si content and critical temperature were gained by theoretical analysis. At the lower primary Si content or lower temperature than critical parameters, the density of the primary crystal silicon is lower than that of the remained Al-Si liquid alloy. When the primary Si content is higher than the critical content and the temperature is higher than the critical temperature, the density of the primary crystal silicon is higher than that of the remained liquid alloy. In the centrifugal force field, the primary crystal silicon in the hypereutectic Al-Si alloys, whose primary Si content is lower than the critical content, is assembled in the lining by the centrifugal force, and the primary crystal silicon in the high Si content hypereutectic Al-Si alloys, whose Si content is higher than the critical content, is assembled in outside and lining. The analysis agrees with the experiments.