Al-P master alloys have been prepared and proved to be an environmental-friendly, high-efficiency and long-lasting modifiers for primary Si in Al-Si alloys.To ensure a good phosphorous modification effect, the Al-P cluster should keep stable in complex Al-Si-X-P melt and then precipitate as AlP phase during the solidification.In this paper, the chemical stability of Al-P cluster was studied systematically for the first time using AIMD simulation, experiments, Ellingham diagram and electronegativity theory.