An AlSi-Mg_2Si self-fluxing brazing filler metal was obtained by activating Al Si brazing filler metal with the Mg_2Si phase and applying a metamorphism treatment of Mg_2Si and Si crystals with Sr and Sm O. A good wetting performance between the modified AlSi-Mg_2 Si and 3003 Al alloys was achieved in the vacuum brazing without Mg vapour as the activator. The melting point of the brazing filler was measured by STA409 Pc differential scanning calorimetry. A scanning electron microscope was used to analyse the microstructure and the component distribution of the brazing filler. The results indicated that the change in morphology of the Mg_2 Si phase was remarkable after metamorphism. The analyses of the microstructure indicated that the Mg_2 Si and Si phases presented small needle-type and granular morphologies. The follows were found to occur: intergranular penetration of Mg and Si on the base metal, a large amount of granular eutectic structure, and a significant grain boundary effect. The shear fracture of the lap joint presented cleavage fracture; in addition, with the decrease of the Mg_2 Si phases, the fracture morphology transformed from a rock candy shape to a steam shape, and cleavage steps appeared. The rock candy-shaped fracture areas enlarged after metamorphism treatment. An AlSi-Mg_2Si self-fluxing brazing filler metal was obtained by activating Al Si brazing filler metal with the Mg_2Si phase and applying a metamorphism treatment of Mg_2Si and Si crystals with Sr and Sm O. A good wetting performance between the modified AlSi-Mg_2 Si and 3003 Al alloys was achieved in the vacuum brazing without Mg vapor as the activator. The melting point of the brazing filler was measured by STA409 Pc differential scanning calorimetry. A scanning electron microscope was used to analyze the microstructure and the component distribution of the brazing The results indicated that the change in morphology of the Mg_2 Si phase was remarkable after metamorphism. The analyzes of the microstructure indicate that the Mg_2 Si and Si phases are small needle-type and granular morphologies. The following were found to occur: intergranular penetration of Mg and Si on the base metal, a large amount of granular eutectic structure, and a significant grain boundary effect. The shear fracture of the lap joint presented cleavage fracture; in addition, with the decrease of the Mg_2Si phases, the fracture morphology transformed from a rock candy shape to a steam shape, and cleavage steps enlarged. after metamorphism treatment.