Al-5Ti-B and Al-5Ti-B-Gd master alloy refiners were fabricated by fluorine salt casting method. The microstructure and phase constitution of the master alloys were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that Al-Ti-B alloy refiner consists of Al3Ti phase and TiB2 phase. After Gd is introduced into the intermediate alloy, Ti2Al20Gd phase appears in the alloy, the size of Al3Ti is significantly reduced, and Ti-Al-Gd phase is found in the edge of Al3Ti phase. At the same time, some independent Ti-Al-Gd phases appear in local areas, which are Ti2Al20Gd phase determined by micro-area electron diffraction analysis. Analysis and calculation results of the high-resolution images of the Ti2Al20Gd/Al structure show that there is no other compound at the junction between the Ti2Al20Gd phase and Al, and Ti2Al20Gd phase has a great difference in atomic space with the α-Al, which cannot be directly used as heterogeneous nucleus. But, after being decomposed in the aluminum melt, the Ti2Al20Gd phase can promote the refinement effect of the refiner. In the Al-Ti-B-Gd master alloy, there are many dispersed Al3Ti particles with a size of less than 1 μm, which can promote the Al-5Ti-B refining effect.