Abstract: Direct observation and analysis of the dynamic process of brazing are essential to the study of the mechanism of vacuum brazing of aluminium.It has been shown by experimental results that the vapourization of Mg in Al-Si-Mg system is closely related to the heating temperature and the vacuum.Large amont of vapourization of magnesium occurs in the temperature range of 500-580℃.Magnesium vapourized in the vacuum chamber purifies the atmosphere as it reacts with the oxidizing agents such as O₂ and H₂O.Then it reacts with Al₂O₃ film to produce porous MgAl₂O₄.Dynamic observation and analysis clearly show that there exists a tunnelling effect of magnesium vapour on the surface film in the process of wetting of the molten Al-Si-Mg brazing alloy on base metal.Therefore,the principal function of magnesium vapour in the process of vacuum brazing of aluminium is its reducing and tunnelling effect.