Abstract: The Al-5Nb-B grain refiner was prepared by fluorinated salt method, and the prepared refiner was added to Al-12Si alloy with different contents (0.0wt%, 1.5wt%, 3.0wt%, 4.5wt%). Optical microscopy and scanning electron microscopy were used to observe the microstructure of Al-12Si alloy with different grain refiners, and the mechanical properties were tested at room temperature in an electronic universal testing machine (D2-02001). The effects of Al-5Nb-B grain refiner on the microstructure and tensile properties of near-eutectic Al-12Si alloy were studied.The results show that the Al-5Nb-B grain refiner prepared by fluorinated salt method mainly contains Al₃Nb particles with an average size of 20 μm and NbB₂ particles with an average size of 5 μm. With the increase of the Al-5Nb-B grain refiner content, the α-Al grain size in Al-12Si alloy decreases from 1864 μm without addition to 396 μm, the tensile strength of as-cast alloy increases from 128 MPa to 145 MPa, and the elongation after fracture increases from 6.9%to 9.8%.The Al-5Nb-B grain refiner prepared by the fluorinated salt method increases the nucleation number of NbB₂ in the Al-12Si alloy melt, effectively refines the α-Al matrix, and improves the mechanical properties of the alloy.