The invention discloses a nucleating agent applied to modification and refinement of hypoeutectic aluminum-silicon alloy and relates to aluminum based alloy. The preparation method comprises the steps: preparing materials according to a mass ratio of Al-B intermediate alloy to Al-Ca intermediate alloy as 5 to (1 to 3); preparing block-shaped Al-B-Ca intermediate alloy; utilizing an in-situ synthesis reaction method and a rapid solidification technology to prepare the nucleating agent applied to modification and refinement of the hypoeutectic aluminum-silicon alloy. The nucleating agent is a thin-stripe-shaped CaB6-Al4Ca/Al ultrafine grain nucleating agent. The nucleating agent disclosed by the invention overcomes the defects that an effect of a nucleating agent in the prior art in refinement of hypoeutectic aluminum-silicon alloy is not obvious, modification treatment and refinement treatment cannot be simultaneously performed, a dispersion degree of particles on a matrix is not ideal,the particles can be gathered and precipitated in melt, and the sizes of the particles are large.