Abstract: In order to improve the surface activation during aluminum alloy vacuum brazing, the Mg2Si was introduced into AlSi filler metal by in situ generation method, consequently a composite filler metal was developed. The metallographic analysis of the filler metal by means of electron probe shows that the introduced Mg2Si spread over the filler metal matrix in massive, skeletal and granular forms. The map scanning method was used to analyze the brazing interface, the results show that a large amount of magnesium dissolved in-Al matrix. The filler metal containing Mg2Si can form a wider diffusion layer. The Mg2Si presented as the forms of granular and short rods in the brazed seam. And it extended radially to the sides of the base material along the α-Al grain boundaries. After the modification of Sr and SmO, the brazed seam became narrower, the radial diffusion layer disappeared, and the compounds were refined with granular shape.