The invention discloses a preparation method of an aluminum-silicon alloy. The preparation method comprises the steps: heating blocky silicon to 1300-1400 DEG C to ensure that the blocky silicon is melted to molten silicon, controlling the temperature of the molten silicon to 760-770 DEG C, and mixing the molten silicon with molten aluminum under a high-purity nitrogen or argon protection atmosphere to prepare molten aluminum-silicon alloy. The prepared molten aluminum-silicon alloy is thin and uniform in silicon phase and uniform in distribution. Silicon is remarkably refined, and crystalline grains are smaller.