[en] Highlight• The addition of 0.5wt.% Sb with a holding time over 60 min leads to the disappearance of twin and the formation of high number-density Al-rich nano-particles in eutectic Si, which act as nucleation precursors and promote the eutectic nucleation in Al–10Si alloy; • A mechanism of precursor nucleation and microstructure transition is proposed, based on the formation of high number-density Al-rich nano-particles; • The existence of AlSb clusters in the Al–10Si–0.5Sb melts is eliminated, while Sb–Si repulsive force and Sb–Sb segregation lead to aggregation of Si atoms and precursors, respectively. The mechanism of refinement of eutectic Si with Sb additions in an Al–10 wt.% Si alloy is studied. The increased holding time designates a transition of the eutectic nucleation temperature from a decreased value to an increased one, accompanied by a morphological transformation of eutectic Si from lamellar to a shorter, rod-like shape. By ab initio molecular dynamics simulation, the chemical short-range order of − 0.002 demonstrates weak affinity in the Al–Sb pair, which does not favor the existence of AlSb clusters in the melts. Sb–Sb segregation and the Sb–Si repulsive force are revealed, which promote the formation of Al-enriched precursors and the accumulation of Si atoms adjacent to precursors, inducing precursor nucleation. Continuous attachment of precursors and individual Si atoms during Si growth results in the high number-density nano-particles and suppresses the formation of twin grains within eutectic Si.