Abstract: As high pure magnesium of the solid state will volatilize when it is heated, the influence of active element Mg content on the performance of brazed joint in the vacuum was researched as brazing aluminum alloy 3A21. The mechanical properties and microstructure of the joint were analyzed with universal testing machine and microscope. By comparison of shear strength of different brazed joints with three kinds of Mg contents it is found that the strength is as high as 78 MPa when the Mg content reaches 235 g/m³. The microstructures indicate that the reinforcing phase is Mg₂Si and the width of brazing zone was the least, 0.06 mm, when the Mg content is 235 g/m³. The Mg content can be controlled in the hearth through obtaining the staying time over 325℃ of different products.